Crystalline gallium nitride containing flourine

ABSTRACT

A method for growing a crystalline composition, the first crystalline composition may include gallium and nitrogen. The crystalline composition may have an infrared absorption peak at about 3175 cm−1, with an absorbance per unit thickness of greater than about 0.01 cm−1. In one embodiment, the composition may have an amount of oxygen present in a concentration of less than about 3×1018 per cubic centimeter, and may be free of two-dimensional planar boundary defects in a determined volume of the first crystalline composition.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/558,048,filed on Nov. 9, 2006, which is a continuation-in-part of applicationSer. No. 11/376,575, filed on Mar. 15, 2006 that is acontinuation-in-part of application Ser. No. 11/010,507, filed Dec. 13,2004, and a continuation-in-part of application Ser. No. 10/329,981,filed Dec. 27, 2002. This application claims priority to and benefitfrom the foregoing, the disclosure of which is incorporated herein byreference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH & DEVELOPMENT

The United States Government may have certain rights in this inventionpursuant to Cooperative Agreement No. 70NANB9H3020, awarded by theNational Institute of Standards and Technology, United States Departmentof Commerce.

BACKGROUND

Field of Technology

Embodiments relate to a method associated with the crystallinecomposition.

Discussion of Related Art

Metal nitride based optoelectronic and electronic devices may becommercially useful. It may be desirable to have metal nitrides withrelatively lower defect levels. Such defects may include threadingdislocations in semiconductor layers of the devices. These threadingdislocations may arise from lattice mismatch of the metal nitride layersto a non-homogeneous substrate, such as sapphire or silicon carbide.Defects may arise from thermal expansion mismatch, impurities, and tiltboundaries, depending on the details of the growth method of the layers.

It may be desirable to have a method of making and/or using a metalnitride that differs from those currently available.

BRIEF DESCRIPTION

In one embodiment, a method may include growing onto a substrate a firstcrystalline composition. The first crystalline composition may includegallium and nitrogen. The crystalline composition may have an infraredabsorption peak at about 3175 cm⁻¹, with an absorbance per unitthickness of greater than about 0.01 cm⁻¹. Or, the first crystallinecomposition may have an amount of oxygen present in a concentration ofless than about 3×10¹⁸ per cubic centimeter, and may be free oftwo-dimensional planar boundary defects in a determined volume of thefirst crystalline composition. The first crystalline composition may beremoved from the substrate so that the first crystalline compositiondefines a seed crystal. A second crystalline composition may be grownonto at least one surface of the first crystalline composition.

Embodiments of the invention may include a method of heating a sourcematerial that is under pressure and in communication with a nucleationcenter. The nucleation center may include a first crystallinecomposition having less than about 5 mole percent of each of phosphorus,arsenic, aluminum, or indium. The first crystalline composition mayinclude at least one grain having a diameter greater than 3 mm, aone-dimensional dislocation density of less than about 1000 per squarecentimeter, and may be free of two-dimensional defects, such as tiltboundaries. The method may include further growing a second crystallinecomposition onto the first crystalline composition. In one aspect, thesecond crystalline composition may include gallium nitride.

In one aspect, the invention also relates to a method forming acrystalline composition comprising gallium nitride, wherein the growthof the crystal is lateral with one embodiment, the lateral growth in the<11-20> direction produces a crystalline composition with large-area(0001) and (000-1) planes. In another embodiment, the lateral growth inthe <10-10> direction produces a crystalline composition with large-area(0001) and (000-1) planes; a second embodiment with lateral growth inthe <000-1> direction producing a crystalline composition withlarge-area (10-10) planes; and a third embodiment with lateral growth inthe <000-1> direction producing a crystalline composition withlarge-area (11-20) planes.

BRIEF DESCRIPTION OF THE DRAWINGS

The same reference numbers may be used throughout the drawings to referto the same or like parts.

FIG. 1 is a schematic cross-sectional representation of a capsule usedfor making a gallium nitride crystalline composition according to anembodiment of the invention.

FIG. 2 is a schematic cross-sectional representation of a pressurevessel used for making a gallium nitride crystalline compositionaccording to an embodiment of the invention.

FIG. 3 is a series of photoluminescence spectra of a crystallinecomposition according to an embodiment of the invention.

FIG. 4 is a schematic illustration of the evolution of dislocations inbulk gallium nitride grown on a c-oriented seed crystal containingdislocations.

FIG. 5 is a schematic illustration of the evolution of tilt boundariesin bulk gallium nitride grown on a c-oriented seed crystal containingtilt boundaries.

FIG. 6 is a schematic illustration of gallium nitride seeds with cutoutsenabling growth of large areas of low-dislocation-density crystallinecompositions even with defective seeds.

FIGS. 7(a)-(c) are schematic illustrations of the edges of galliumnitride wafers with FIG. 7(a) a simply-ground edge; FIG. 7(b) achamfered edge; or FIG. 7(c) a rounded edge.

FIG. 8 shows the infrared spectrum of an exemplary bulk gallium nitridesubstrate produced in accordance with an embodiment of the invention.

FIG. 9 shows the approximate dislocation density as a function ofthickness for a gallium nitride film grown by HVPE.

FIG. 10 is a photograph of a crystalline composition grown by a methodin accordance with an embodiment of the invention.

FIG. 11 is a photograph of another crystalline composition grown by amethod in accordance with an embodiment of the invention.

FIG. 12 is a plot showing the dependence of laser diode lifetime ondislocation density.

DETAILED DESCRIPTION

Embodiments may relate to a crystalline composition having determinedcharacteristics. Embodiments may relate to a method associated withmaking and/or using the crystalline composition. Also provided are oneor more wafers formed from the crystalline composition, and anelectronic device formed from the wafer.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termsuch as “about” is not limited to the precise value specified. In someinstances, the approximating language may correspond to the precision ofan instrument for measuring the value.

Crystalline composition and quasi-crystalline composition may includematerial where atoms form a uniform periodic array. A quasi-crystallinecomposition may have a predetermined number of grains per unit area.Crystalline composition defects may be present in each grain, or may bea grain boundary that defines a grain and may be present in an amountof, for example, more than one but less than about 10,000 defects persquare centimeter, or in a range of up to about 10¹⁶ defects per cubiccentimeter. Polycrystalline material includes a plurality of randomlyoriented grains where each grain may include a single crystal, where theplurality of grains are present at more than about 10¹⁶ grain boundariesper cubic centimeter.

Crystalline composition defects refers to one or more of point defects,such as vacancies, interstitials, and impurities; one-dimensional lineardefects, such as dislocations (edge, screw, mixed); two-dimensionalplanar defects, such as tilt boundaries, grain boundaries, cleavagepoints and surfaces; and three-dimensional extended defects, such aspores, pits, and cracks. Defect may refer to one or more of theforegoing unless context or language indicates that the subject is aparticular subset of defect. “Free of two-dimensional defects” may beused interchangeably with “free of tilt boundaries,” meaning that thecrystalline composition may have tilt boundaries at an insubstantiallevel, or with a tilt angle such that the tilt boundaries may not bereadily detectable by TEM or X-ray diffraction; or, the crystallinecomposition may include tilt boundaries that are widely separated fromone another, e.g., by at least 1 millimeters or by a greater, andspecified, distance. Thus, “free” may be used in combination with aterm, and may include an insubstantial number or trace amounts whilestill being considered free of the modified term, and “free” may includefurther the complete absence of the modified term.

As used herein, the term “group III nitride semiconductor crystals” or“crystalline composition” or “crystalline composition comprising galliumnitride” refers to GaN, AlN, InN, AlGaN, InGaN and their alloys thereof,as represented by Al_(x)In_(y)Ga_(1-x-y)N, where 0<=x<=1, 0<=y<=1 and0<=x+y<=1.

As used herein, the term “crystalline composition comprising galliumnitride” may be used interchangeably with “a crystal comprising galliumnitride.”

According to embodiments of the invention, a crystalline compositionfree of two-dimensional defects, such as grain and tilt boundaries, maybe synthesized and grown from a single nucleus or from a seed crystal.The crystalline composition has three dimensions as represented by x, y,w characters, with w as the thickness, and x and y the dimensions of thecrystal plane perpendicular to w. For a round or circular crystal,x=y=the diameter of the crystalline composition. The grown crystallinecomposition may have a size of 20 millimeters (mm) or greater in atleast one dimension x or y. The crystalline composition may have one ormore grains, and the grains may have determined characteristics orattributes as disclosed herein.

In one embodiment, the crystalline composition may be n-type,electrically conductive, opaque, free of lateral strain and free oftwo-dimensional planar boundary defects, and may have a one-dimensionallinear dislocation density of less than about 10,000 cm⁻². In oneembodiment, the dislocation density may be less than about 1000 cm⁻², orless than about 100 cm⁻². The two-dimensional planar boundary defectsmay include, for example, tilt boundaries.

In one embodiment, the crystalline composition may be p-type; inanother, it may be semi-insulating. With reference to the p-typematerial, the crystalline composition may function as a p-typesemiconductor at about room temperature, at a temperature in a range ofless than about 300 Kelvin (K), in a range of from about 300 K to about250 K, less than about 250 K, in a range of from about 250 K to about100 K, or less than about 100 K. In one embodiment, the crystallinecomposition may be magnetic, may be luminescent, or may be both. Thecrystalline composition may be one or more of opaque, opticallyabsorbing, and/or black. In one embodiment, an opaque crystallinecomposition may be an undoped crystalline composition; particularly, theopaque crystalline composition may be free of magnesium. Black, as usedherein, is specifically distinguished from dark grey, dark blue, darkbrown, or other color and has no predominant hue.

In one embodiment, the crystalline composition may include hydrogen in aform that results in an infrared absorption peak near 3175 cm⁻¹, with anabsorbance per unit thickness greater than about 0.01 cm⁻¹.

The crystalline composition may contain up to about 5 mole percentboron, aluminum, indium, phosphorus, and/or arsenic. In one embodiment,the crystalline composition may contain boron, aluminum, indium,phosphorus, and/or arsenic in an amount in a range of from about 0.1mole percent to about 0.25 mole percent, from about 0.25 mole percent toabout 1 mole percent, from about 1 mole percent to about 2 mole percent,or from about 2 mole percent to about 5 mole percent. In one embodiment,the crystalline composition may be essentially free of boron. In oneembodiment, the crystalline composition may be essentially free ofaluminum. In one embodiment, the crystalline composition may beessentially free of indium. In one embodiment, the crystallinecomposition may be essentially free of phosphorus. In one embodiment,the crystalline composition may be essentially free of arsenic. In oneembodiment, the crystalline composition may be essentially free ofanother group V element. In one embodiment, the crystalline compositionmay be gallium nitride and may be essentially free of another group IIImetal, apart from gallium.

In one embodiment, the crystalline composition may be doped with atleast one of Be, C, O, Mg, Si, H, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,Zn, Ge, Zr, or Hf. In one embodiment, the crystalline composition may bedoped with at least one rare earth metal. If present, the dopant may beat a concentration in a range of up to about 10¹⁶ cm⁻³, from about 10¹⁶cm⁻³ to about 10²¹ cm⁻³, or greater.

A relatively large gallium nitride crystalline composition may be grownby temperature gradient recrystallization at high-pressure and hightemperature in a superheated fluid solvent. The crystalline compositionmay be a true single crystal, i.e., it does not have any grainboundaries whatsoever. Other crystalline compositions in accordance withan embodiment of the invention may be free of tilt boundaries, that is,they may have an insubstantial number of tilt boundaries, or may have notilt boundaries whatsoever.

These gallium nitride crystalline compositions may be grown bytemperature-gradient recrystallization in a superheated fluid or asupercritical fluid. A suitable fluid may be nitrogen-containing and mayinclude one or more of ammonia, hydrazine, triazine, methylamine,ethylenediamine, melamine, or another nitrogen-containing material. Inone embodiment, the nitrogen-containing fluid consists essentially ofammonia.

The source material may include gallium and nitrogen, which may be inthe form of, for example, gallium nitride crystalline powder. Otherforms of source material may be used, for example, single crystal orpolycrystalline AlN, amorphous gallium nitride or a gallium nitrideprecursor such as gallium metal or a gallium compound. Single crystal orpolycrystalline AlN can be grown by a number of methods known in theart. Other forms of AlN can also be used, for example, amorphous AlN oran AlN precursor such as Al metal or an Al compound. In embodiments witha temperature gradient, it may be that the source material may includeone or more particles that may be sufficiently large in size so as notto pass through the openings in a baffle, described below, thatseparates the source region, where the source material may be located,from the crystalline composition growth region, where a nucleationcenter may be located, of a chamber or capsule, as discussed in moredetail below. In one embodiment for the growth of non-GaN nitrides, theoxide content of the source material is kept at less than about 1 weightpercent.

Nucleation for gallium nitride growth may be induced on a growth portionof the capsule at a nucleation center without a seed crystal, such as aportion of the container wall. Alternatively, a seed crystal may beused.

Suitable seed crystals may be GaN-based, or may be non-GaN-based. A seedcrystal formed entirely from the same nitride may be used for ease ofcontrol and because the quality of the grown crystalline composition maybe relatively higher. In one embodiment for the growth of AlN,nucleation can be induced on the crystal growth portion of the cell witha non-AlN seed crystal such as silicon carbide, but the process iseasier to control if an AlN seed crystal is provided. Suitable GaN-basedseed crystals may include a free-standing gallium nitride film grown byat least one of HVPE, sublimation, or metal organic chemical vapordeposition (MOCVD), or by a crystalline composition grown in asuperheated fluid in a previous run. A variety of types of AlN seedcrystals may be used for growing AlN, including an epitaxial AlN layeron a non-AlN substrate such as sapphire or SiC, a free-standing AlN filmgrown by HVPE or MOCVD, an AlN crystal grown bysublimation/recondensation, or a crystal grown in a supercritical fluidin a previous run.

If a seed crystal that is not entirely formed from gallium nitride isnot used, a suitable non-GaN seed crystal may include sapphire orsilicon carbide. In one embodiment, the non-GaN-based seed crystal maybe pre-coated with a layer of gallium nitride on a growth surface.Suitable coated seed crystals may include an epitaxial gallium nitridelayer on a non-GaN substrate. Whether GaN-based or non-GaN-based, theseed crystal may include an amount of fluorine greater than about 0.04ppm, or in a range of from about 0.04 to about 1 ppm fluorine. The seedcrystal may include an amount of chlorine greater than about 0.04 ppm,or in a range of from about 0.04 to about 1 ppm chlorine. In oneembodiment, the seed crystal is essentially halogen-free.

The seed crystal may be larger than 1 millimeter in at least onedimension x or y and of high quality being free of tilt boundaries andhaving a dislocation density in a range of less than about 10⁸ cm⁻². Inone embodiment, the seed crystal may have a dislocation density in arange of less than about 10⁵ cm⁻². The character and attributes of theseed crystal may directly impact the character and attributes of thecrystalline composition grown thereon.

The seed may have any crystallographic orientation, as growth may occuron all exposed gallium nitride surfaces. Gallium nitride crystallinecompositions grown from seeds may terminate predominantly by (0001),(0001), and (1100) facets, and all these orientations may be suitablefor seed surfaces. The (1120) surfaces may be fast growing in theinventive method, and also constitute favorable seed surfaceorientations. In one embodiment, the crystallographic orientation of thegallium nitride crystalline compositions that may be grown may be withinabout 10° of one of the (0001) orientation, the (0001) orientation, the(1010) orientation, the (1120) orientation, and the (1011) orientation.In one embodiment, the orientation of the grown gallium nitridecrystalline compositions may be within about 5° of one of theseorientations. A standard metric for the crystallinity of as-growngallium nitride crystalline compositions or of gallium nitride wafersmay be provided by x-ray diffraction rocking curve measurements of the(0002) reflection. The full width at half maximum (FWHM) of the (0002)diffraction intensity versus ω of gallium nitride crystallinecompositions and wafers of the inventive method may be less than about50 arc-seconds, less than about 30 arc-seconds, less than about 20arc-seconds, or less than 15 arc-seconds.

With reference to the seed crystals, the seed crystals may have adislocation density below 10⁴ cm⁻² and may be free of tilt boundaries.The use of low-defect seed crystals may result in a grown crystallinecomposition that similarly has a relatively low dislocation density andrelatively low density of other types of defects. In one embodiment, thegallium nitride seed crystals contain one or more tilt boundaries, evenif no grain boundaries are present. In one embodiment, the nature andcharacter of the seed crystal affects and controls the nature andcharacter of a crystal grown on a surface of the seed crystal.

A gallium nitride crystalline composition with a one-dimensionaldislocation density that is less than about 10⁴ cm⁻² and that is freefrom two-dimensional defects, such as tilt boundaries, may be grown fromseed crystals with a dislocation density in a range of from about 10⁵cm⁻² to about 10⁸ cm⁻² and that is free from two-dimensional defects,such as tilt boundaries, by the following procedure:

By suitable control of the source material, solvent fill, mineralizerconcentration, temperature, and temperature gradient, growth on the seedmay occur in both the c direction (that is, <0001> and <0001>, along thec-axis) and perpendicular to the c direction. The dislocation density410 in bulk gallium nitride grown in the c-direction may be reducedsignificantly. For example, growth of a 300-800 μm thick layer above ac-oriented seed crystal 402 containing approximately 10⁷ dislocationscm⁻² results in a gallium nitride crystalline composition withapproximately 1-3×10⁶ dislocations cm⁻² in the region above the seed404, as shown in FIG. 4.

However, the bulk gallium nitride grown laterally 406 with respect to ac-oriented seed crystal 402 has fewer than 10⁴ dislocations cm⁻², fewerthan 10³ dislocations cm⁻², and even more fewer than 100 dislocationscm⁻², as illustrated in FIG. 4. Tilt boundaries 510 that may be presentin a c-oriented seed crystal 502 may propagate during growth in the cdirection, resulting in a grain structure in bulk gallium nitride grownabove 504 the seed that may be similar to that in the seed 502, asillustrated schematically in FIG. 5. However, tilt boundaries 510 mayradiate outward in bulk gallium nitride that may be grown laterally, forexample, by growth in the m-direction or in the a-direction, resultingin progressively larger domains 520 that may be free of tilt boundaries510 as the crystalline composition becomes larger, as illustrated inFIG. 5. The position of the tilt boundaries 510 may be determined byx-ray diffraction, x-ray topography, or simple optical reflection, and anew seed crystal may be cut from the laterally-grown gallium nitridethat may be entirely free of tilt boundaries. Bulk gallium nitride grownfrom this new seed crystal may be free of tilt boundaries and may have adislocation density below 10⁴ cm⁻², below 10³ cm⁻², and even more below100 cm⁻². While this discussion assumes a c-oriented seed crystal, seedcrystals of other orientations may be employed, such as within about 10°of one of the (0001) orientation, the (0001) orientation, the (1010)orientation, the (1120) orientation, and the (1011) orientation. Thedislocation density similarly may be reduced by lateral growth from theoriginal seed crystal and tilt boundaries may radiate outward, enablingseed crystals that may be free of tilt boundaries and may have adislocation density below 10⁴ cm⁻², below 10³ cm⁻², or, in oneembodiment, below 100 cm⁻².

Relatively large areas of gallium nitride with a one-dimensional lineardislocation density below 10⁴ cm⁻², below 10³ cm⁻², and even more below100 cm⁻² may be prepared using seeds with higher dislocation densitiesby the following procedure. Holes, cutouts, or zigzag patterns may beplaced in the seeds by means of cutting by a laser, for example.Examples of such seeds 610 may be shown in FIG. 6. The holes, cutouts,or other patterns may be round, elliptical, square, or rectangular, forexample. In one embodiment, shown in FIG. 6, the long dimensions ofslots 602 or zigzag cuts 604 may be oriented approximately parallel to(1010) (m plane). In this orientation a steady growth front may occur,filling in the slot 602 or space 606 smoothly. In this way lateralgrowth 612 can take place in the central portion of a crystallinecomposition rather than just at the periphery, producing large domains608 of very low dislocation density, below 10⁴ cm⁻², material even whenusing seeds with a relatively high dislocation density, above 10⁶ cm⁻².This process may be repeated. A crystalline composition grown by themethod described above may contain regions of moderately low and verylow dislocation densities. Regions of the crystalline composition withhigher dislocation densities may be cut out and the crystallinecomposition used again as a seed. Lateral growth 612 may again fill inthe cut out 602 areas with very low dislocation density material 608. Inthis way large area gallium nitride crystalline compositions can beproduced that have dislocation densities less than 10⁴ cm⁻², and lessthan 100 cm⁻², over greater than 80 percent of their area. Thesecrystalline compositions may contain tilt boundaries at the regions ofcoalescence in the laterally-grown material, but the separation betweenthe tilt boundaries can be made larger than about 2 millimeters (mm),2.75 mm, 3 mm, 5 mm, 10 mm, 18 mm, 25 mm, or greater than 25 mm.

By these lateral growth methods, either along the periphery of a seedcrystal or with a patterned seed crystal, it may be possible to producecrystalline compositions with grain boundaries spaced 2 millimetersapart, or greater. In one embodiment, at least one dimension x or y ofthe single crystal grain may be in a range of from about 2 mm to about2.75 mm, from about 2.75 to about 3 mm, from about 3 mm to about 5 mm,from about 5 mm to about 10 mm, from about 10 mm to about 25 mm, of from25 mm to 600 millimeters in at least one dimension x or y. Use of awafer sliced from such a crystalline composition as a substrate enablesfabrication of large-area homoepitaxial gallium nitride-based electronicor optoelectronic devices that may be free of tilt boundaries.

With reference to the thickness of the crystalline composition grown inaccordance with an embodiment of the invention, the thickness may begreater than about 100 micrometers. In one embodiment, the thickness maybe in a range of from about 100 micrometers to about 0.3 mm, from about0.3 mm to about 1 mm, from about 1 mm to about 1.5 mm, from about 1.5 mmto about 10 mm, or greater than about 10 millimeters.

The source material and one or more seeds, if used, may be placed in apressure vessel or capsule that may be divided into at least two regionsby means of a porous baffle.

FIG. 1 illustrates an exemplary capsule 100. The capsule 100 includes awall 102, which can be sealed to surround a chamber 104 of the capsule100. The chamber may be divided into a first region 108 and a secondregion 106 separated by a porous baffle 110. During crystallizationgrowth the capsule 100 may include a seed crystal 120 or othernucleation center and a source material 124 separated from each other bythe baffle 110. The source material 124 and the seed crystal 120 may bepositioned in the second region 106 and the first region 108,respectively, for example. The capsule 100 also may include a solventmaterial 130. During the growth process, described below, a growncrystalline composition 132 may be grown on the seed crystal 120 and thesolvent may be in a superheated state.

The baffle 110 may include, for example, a plate with a plurality ofholes in it, or a woven metal cloth. The fractional open area of thebaffle 110 may be in a range of from about 1 percent to about 50percent, from about 2 percent to about 10 percent, from about 1 percentto about 2 percent, or from about 10 percent to about 50 percent.Transport of nutrient from the source material 124 to the seed crystal120 or grown crystalline composition 132 may be optimized in the solventas a superheated fluid if the colder portion of the capsule 100 may beabove the warmer portion, so that self-convection stirs the fluid. Insome solvents, the solubility of gallium nitride may increase with anincrease in temperature. If such a solvent is used, the source material124 may be placed in the lower and warmer portion of the capsule and theseed crystal 120 may be placed in the upper or colder portion of thecapsule.

The seed crystal 120 may be hung, for example, by a wire 150 fastenedthrough a hole drilled through the seed, so as to allow crystallinecomposition growth in all directions with a minimum of interference fromwall 102, wire 150, or other materials. A suitable hole may be formed bya laser, or by a diamond drill, an abrasive drill, or an ultrasonicdrill. The seed crystal 120 may be hung by tying a wire around an end ofthe seed.

In the case of some solvents, however, the solubility of gallium nitridemay decrease with an increase in temperature. If such a solvent is used,the seed crystal 120 may be placed in the lower and warmer portion ofthe capsule and the source material 124 may be placed in the upper andcolder portion of the capsule. The source material 124 may be placed ina porous basket 140 displaced from the baffle 110 rather thanimmediately contacting the baffle 110, as the latter arrangement mayimpede transport of fluid and nutrient through the baffle 110.

A mineralizer may be added to the capsule 100, in order to increase thesolubility of gallium nitride in the solvent, either together with thesource material 124 or separately. The mineralizer may include at leastone of (i) nitrides, such as alkali and alkaline-earth nitrides, andparticularly Li₃N, Mg₃N₂, or Ca₃N₂; (ii) amides, such as such as alkaliand alkaline-earth amides, and particularly LiNH₂, NaNH₂, and KNH₂;(iii) urea and related compounds, such as metal urea complexes; (iv)nitrogen halides, such as ammonium salts, and particularly NH₄F andNH₄Cl; (v) rare earth halides, rare earth sulfides, or rare earthnitrate salts, such as CeCl₃, NaCl, Li₂S, or KNO₃; (vi) azide salts,such as alkaline azides, and particularly NaN₃; (vii) other Li salts;(viii) combinations of two or more of the above; (ix) organicderivatives of one or more of the above, such as alkylammonium halide,particularly triphenylphosphonium chloride; or (x) compounds formed bychemical reaction of at least one of the above with gallium and/orgallium nitride. In one embodiment, the mineralizer is an acidicmineralizer, and may be entirely free of a basic mineralizer. An acidicmineralizer may produce a hydronium ion under process conditions.

In one embodiment, ammonia may be employed as the superheated fluidsolvent and at least one of hydrogen halide, ammonium halide, galliumhalide, gallium tri halide, or a compound produced by chemical reactionsbetween the halides and one or more of ammonia (NH₃), gallium, orgallium nitride may be employed as the mineralizer. Suitable halides mayinclude fluorine, chlorine, or a combination of fluorine and chlorine.

The combination with a mineralizer may provide a relatively highsolubility of gallium nitride while not being overly corrosive to thecapsule, particularly when the capsule may include silver. In this casethe effective solubility of gallium nitride may decrease withtemperature. The gallium nitride may undergo a chemical reaction withthe mineralizer and solvent to form a complex including gallium halide,ammonium ions, and ammonia, and the complex may be soluble insuperheated fluid, such as ammonia. A suitable complex may includegallium fluoride. Formation of the complexes may be reversible, with anequilibrium constant for formation that decreases with temperature sothat formation of free gallium nitride may be favored at highertemperature and the effective solubility of gallium nitride decreaseswith temperature. After ending a crystalline composition growth run withthis chemistry, the capsule may be filled with white needle-shapedcrystals. X-ray diffraction analysis indicates that the crystallinecompositions may include GaF₃(NH₃)₂ and (NH₄)₃GaF₆, whose structures maybe known from the literature.

In another embodiment, ammonia may be employed as the superheated fluidsolvent and at least one of hydrogen chloride, ammonium chloride,gallium chloride, gallium trichloride, or a compound produced bychemical reactions between HCl, NH₃, Ga, and gallium nitride, may beemployed as the mineralizer. In this case the effective solubility ofgallium nitride may increase with temperature.

Optionally, a dopant source may be also added to provide a determinedtype of crystalline composition. Examples of such determined types mayinclude n-type, semi-insulating, p-type, magnetic, luminescent, oroptically absorbing gallium nitride crystals. Dopants may be added tomodify the bandgap. Adventitious impurities such as oxygen or carbon mayotherwise normally render the crystalline compositions n-type. Dopantssuch as oxygen, silicon, beryllium, magnesium, Ge (n-type), or Zn(p-type), may be added to the source gallium and/or nitrogen.Alternatively, the dopants may be added as metals, salts, or inorganiccompounds, such as Si, Si₃N₄, InN, SiCl₄, AlCl₃, InCl₃, BeF₂, Mg₃N₂,MgF₂, PCl₃, Zn, ZnF₂, or Zn₃N₂. Aluminum, arsenic, boron, indium, and/orphosphorus may be present at levels up to about 5 mole percent, wherethe amount is calculated either individually or collectively. Suchadditions may have the effect of increasing or decreasing the bandgapwith respect to pure gallium nitride. Such doped crystallinecompositions may be referred to herein as gallium nitride, even thoughthey may contain significant levels of another material. Gallium nitridecrystalline compositions with total dopant concentrations below about10¹⁵ cm⁻³ to about 10¹⁶ cm⁻³ may be semi-insulating. However, theconcentration of unintentional impurities may be higher than 10¹⁶ cm⁻³and the crystalline compositions may be n-type. Semi-insulating galliumnitride crystalline compositions may be obtained by doping with at leastone of Sc, Ti, V, Cr, Mn, Fe, Co, Ni, or Cu. In one embodiment,semi-insulating gallium nitride crystalline compositions may be producedby doping with one or both of iron or cobalt.

Magnetic gallium nitride crystalline compositions may be obtained bydoping with certain transition metals, such as, but not limited to,manganese. Luminescent gallium nitride crystalline compositions may beobtained by doping with one or more transition metals or with one ormore rare-earth metals. Suitable luminescent dopants may include one ormore of Ti, V, Cr, Mn, Fe, Co, Ni, Zr, Hf, Pr, Eu, Er, or Tm. Thetransition-metal or rare-earth dopants may be additives in the sourcematerial, or as elemental metal, metal salts, or inorganic compounds. Inone embodiment, the additives may include one or more of Fe, Co, CoF₂,CrN, or EuF₃, either alone or in combination with one or more additionaldopants, such as O, Si, Mg, Zn, C, or H. Such additives may be presentin concentrations in a range of from about 10¹⁵ cm⁻³ to about 10²¹ cm⁻³in the source material. Depending on the identity and concentration ofthe additive, the crystalline composition may be opaque or may beoptically absorbing; e.g., black. For example, heavily Co-doped galliumnitride crystalline compositions may be black in color and may produceno visible photoluminescence in response to irradiation with a nitrogenlaser.

In one embodiment, the impurity levels in the raw materials (sourcematerial, mineralizer, and solvent) and capsules may be limited toappropriately low levels to keep the concentration of undesired dopants,such as oxygen, to an acceptable level. For example, an oxygenconcentration below 3×10¹⁸ cm⁻³ in the grown crystalline compositionsmay be achieved by holding the total oxygen content in the raw materialsand capsule below 15 parts per million, with respect to the weight ofthe final crystal, and an impurity level below 3×10¹⁷ cm⁻³ may beachieved by holding the total oxygen content in the raw materials andcapsule below 1.5 parts per million.

The presence of oxygen in the system (raw materials, vessel) may bedeleterious to the ammonothermal growth of nitrides under somecircumstances. Oxygen contents above 10¹⁸ cm³, above 10¹⁹ cm⁻³, and evenabove 10²⁰ cm⁻³ have been observed in some ammonothermally-grown GaN. Inone embodiment of the invention, it is noted that the role of oxygen ismore deleterious in the case of growth of both AlN and InN than it iswith GaN. From the free energy of formation of pure substances, the easewith which ammonia may reduce the group III metal nitride may beillustrated in Table I below. Data is from I. Barin, with a correctionfor the enthalpy of formation of GaN [Ranade et al., J. Phys. Chem. B104, 4060 (2000)].

TABLE 1 Free energies of reaction ΔG° for selected oxide-reductionreactions (kJ/mol). Al₂O₃ + 2NH₃ = Ga₂O₃ + 2NH₃ = In₂O₃ + 2NH₃ = T (K)2AlN + 3H₂O 2GaN + 3H₂O 2InN + 3H₂O 800 269 16 127 900 252 1 111 1000236 −14 96 1100 219 −29 80

As illustrated, at temperatures of approximately 900 K and above,ammonia can readily reduce gallium oxide to gallium nitride inqualitative agreement with the observation that GaN may be grownammonothermally even when large quantities of oxygen or oxide arepresent in the raw materials. However, ammonia does not readily reduceeither aluminum oxide or indium oxide at any of these temperatures,which is surprising in the sense that the thermodynamics of a givenreaction type may vary monotonically as one passes down a column in thePeriodic Table (Al—Ga—In). Accordingly, desired results may be obtainedbased on the oxygen content of the raw materials, which may be less than1%. Even further, the results may differ when the oxygen content is lessthan 0.1%, less than 0.01%, less than 0.001% (10 ppm), or less than 1ppm.

In one embodiment, in order to reduce the concentration of undesireddopants, such as oxygen, to an acceptable level, one or more getters maybe also added to the capsule. For non-fluoride mineralizers, such asNH₄Cl, suitable getters include alkaline earth metals, Sc, Ti, V, Cr, Y,Zr, Nb, Hf, Ta, W, rare earth metals, and their nitrides or halides.When NH₄F, HF, GaF₃ (or their chlorine equivalents) and/or theirreaction products with NH₃, gallium, and gallium nitride, may be used asmineralizers, also referred to herein as acid mineralizers, highlyreactive metals may tend to form metal halide, which may be unreactivewith water or oxygen in the system. However, compounds of metals mayhave the property that the free energy of reaction of the metal fluoridewith water to form the metal oxide and HF may be more negative undercrystalline composition growth conditions than the correspondingreaction of GaF₃ with water may be used as getters. Suitable getters foruse with acid fluoride mineralizers include CrF₃, ZrF₄, HfF₄, VF₄, NbF₅,TaF₅, and WF₆.

The capsule 100 may be filled with a solvent 130 that may include asuperheated fluid under processing conditions, such as, for example,ammonia, hydrazine, methylamine, ethylenediamine, melamine, or othernitrogen-containing fluid. In one embodiment ammonia may be employed asthe solvent 130. Of the free volume in the capsule, i.e., the volume notoccupied by the source material, seed(s), and baffle), between 25percent and 100 percent, or between 70 percent and 95 percent, may befilled with solvent 130 and the capsule 100 may be sealed.

Depending upon the concentration of the mineralizer dissolved into thesuperheated fluid solvent, under crystalline composition growthconditions the superheated fluid solution may be either supercritical ormay be subcritical. For example, ammonia has a critical temperature andpressure of 132 degrees Celsius and 113 bar, respectively. Thecorresponding quantities for NH₄F may be expected to be similar to thevalues for NH₄Cl, which may be about 882 degrees Celsius and 1635 bar. Asolution of NH₄F in ammonia may be expected to have a critical point ata temperature and pressure intermediate between the criticaltemperatures and pressures of the constituents NH₄F and ammonia. Thepresence of gallium-containing complexes in the solution may furthermodify the equation of state and critical point of the superheatedfluid.

In one embodiment, the mineralizer may be present at a concentrationbetween 0.5 and 5 mole percent with respect to the solvent. Acidmineralizers, for example, NH₄F and NH₄Cl, may differ in effectivenessbased on the concentration. Differing effects may be observed atconcentrations above any one of 0.5 mole percent, 1 mole percent, 2 molepercent, 5 mole percent, 10 mole percent, 20 mole percent, 50 molepercent, or more in ammonia. In the case of NH₄F, the concentration ofdissolved gallium nitride, that is, the concentration of gallium presentin complexes that may be believed to be dissolved under crystallinecomposition growth conditions, may be approximately proportional to themineralizer concentration at values at least as high as 25 mole percent,and that gallium nitride crystalline composition growth may be veryeffective under these conditions. The use of mineralizer concentrationsabove 20 mole percent in ammonia has the added benefit of reducing thepressure of the solvent at a given fill level, thereby reducing themechanical demands on the pressure vessel.

The capsule 100 may be cooled to a temperature at which the solvent 130may be either a liquid or solid. Once the capsule 100 may besufficiently cooled, a solvent source may be placed in fluidcommunication with the open chamber of the capsule 100 and solvent maybe introduced into the chamber, which may be open at this point, byeither condensation or injection. After a desired amount of solvent 130may be introduced into the open chamber, the chamber may be sealed.Pinching off or collapsing a portion of the wall 102 to form a weld mayseal the chamber.

The sealed capsule 100 may be placed in a vessel capable of generatingtemperatures in a range of greater than about 550 degrees Celsius. Thetemperature may be in a range of from about 550 degrees Celsius to about650 degrees Celsius, from about 650 degrees Celsius to about 750 degreesCelsius, from about 750 degrees Celsius to about 900 degrees Celsius orgreater than about 900 degrees Celsius. The pressure may be in a rangeof from about 5 kbar to about 10 kbar, from about 10 kbar to about 15kbar, from about 15 kbar to about 20 kbar, from about 20 kbar to about50 kbar, or greater than about 50 kbar. The capsule may be formed frommaterials, and structurally designed, to be capable of functioning atthe elevated temperature and pressure, while filled with the rawmaterials, for a determined length of time. Capsules that are capable ofreceiving the raw materials, but are unable to remain sealed duringprocess conditions are not suitable. Likewise, capsules that are formedfrom, or lined with, material that negatively impact the reactionproduct to a determined degree are not suitable for use in someembodiments.

FIG. 2 illustrates a pressure vessel 210 housing the enclosed capsule100. The pressure vessel 210 illustrated in FIG. 2 may include ahydraulic press with a die.

The pressure vessel 210 may include a pressure medium 214 enclosed bycompression die 204 and top and bottom seals 220 and 222. The pressuremedium may be, for example, NaCl, NaBr or NaF.

The pressure vessel 210 includes a wattage control system 216 forcontrolling the heating of the capsule 100. The wattage control system216 includes a heating element 218 to provide heating to the capsule100, and a controller 222 for controlling the heating element 218. Thewattage control system 216 also includes at least one temperature sensor224 proximate to the capsule 100 for generating temperature signalsassociated with the capsule 100.

The pressure vessel 210 may be arranged to provide a temperaturedistribution, i.e., the temperature as a function of the position withinthe capsule chamber, within the capsule chamber, including a temperaturegradient within the capsule 100. In one embodiment, the temperaturegradient may be achieved by placing the capsule 100 closer to one end ofthe cell (the region within the pressure vessel 210) than the other.Alternatively, providing at least one heating element 218 having anon-uniform resistance along its length may produce the temperaturegradient.

Non-uniform resistance of the at least one heating element 218 may beprovided, for example, by providing at least one heating element 218having a non-uniform thickness, by perforating the at least one heatingelement 218 at selected points, or by providing at least one heatingelement 218 that includes a laminate of at least two materials ofdiffering resistivity at selected points along the length of the atleast one heating element 218. In one embodiment, the at least onetemperature sensor 224 includes at least two independent temperaturesensors provided to measure and control the temperature gradient betweenthe opposite ends 230, 232 of the capsule 100. In one embodiment,closed-loop temperature control may be provided for at least twolocations within the cell. The at least one heating element 218 may alsoinclude multiple zones which may be individually powered to achieve thedesired temperature gradient between two ends of the capsule 100.

The capsule 100 may be heated to one or more growth temperatures. Thegrowth temperatures may be in a range of greater than about 550 degreesCelsius. The temperature may be in a range of from about 550 degreesCelsius to about 650 degrees Celsius, from about 650 degrees Celsius toabout 750 degrees Celsius, from about 750 degrees Celsius to about 900degrees Celsius, or greater than about 900 degrees Celsius. The heatingmay be performed at an average ramp rate in a range of from about 1degrees Celsius/hr to about 1000 degrees Celsius/hr. A temperaturegradient may be present in the capsule, due to asymmetric placement ofthe capsule in the cell, non-symmetric heating, or the like, asdescribed above with respect to the pressure cell 210. This temperaturegradient may create supersaturation throughout the heating sequence, andmay promote spontaneous nucleation.

In one embodiment, the temperature gradient at the growth temperaturemay be initially held small, less than about 25 degrees Celsius and lessthan about 10 degrees Celsius, for a period in a range of from about 1minute and 2 hours, in order to allow the system to equilibrate in anequilibrium stage. The temperature gradient as used in this applicationmay be the difference in the temperature at the ends of the capsule, forexample, where the control thermocouples may be located. The temperaturegradient at the position of the seed crystal 120 or nucleation centerwith respect to the temperature at the position of the source material124 may be likely to be somewhat smaller.

Optionally, the temperature gradient may be set in the equilibrium stageto be opposite in sign to that where crystalline composition growthoccurs on the nucleation center (i.e., so that etching occurs at thenucleation center and growth occurs on the source material) so as toetch away any spontaneously-nucleated crystalline compositions in theregion of the capsule where the nucleation center may be provided thatmay have formed during heating. In other words, if the crystallinecomposition growth occurs for a positive temperature gradient, thetemperature gradient may be set to be negative, and vice versa.

After this equilibration period, a growth period may be provided wherethe temperature gradient may be increased in magnitude and has a signsuch that growth occurs at the seed crystal at a greater rate. Forexample the temperature gradient may be increased at a rate in a rangeof from about 0.01 degrees Celsius/hr to about 25 degrees Celsius/hr, toa larger value where growth may be faster.

During the crystalline composition growth, the temperature gradient maybe held at a temperature in a range of greater than about 550 degreesCelsius. The temperature may be in a range of from about 550 degreesCelsius to about 650 degrees Celsius, from about 650 degrees Celsius toabout 750 degrees Celsius, from about 750 degrees Celsius to about 900degrees Celsius, or greater than about 900 degrees Celsius. The holdtemperature may be adjusted upward and/or downward during growth.Optionally, the temperature gradient may be changed to have a signopposite to the sign where growth may occur at the seed crystal. Thesign of the gradient may be reversed one or more additional times toalternately etch away spontaneously-formed nuclei and promote growth onone or more nucleation centers or seed crystalline compositions 120. TheHPHT conditions may be maintained for a length of time sufficient todissolve a substantial portion of the source gallium nitride and toprecipitate onto at least one gallium nitride crystal, gallium nitrideboule, or gallium nitride crystalline composition seed.

At the conclusion of the growth period the temperature of the capsulemay be ramped down at a ramp rate in a range of from about 1 degreesCelsius/hr to about 100 degrees Celsius/hr, from about 100 degreesCelsius/hr to about 300 degrees Celsius/hr, from about 300 degreesCelsius/hr to about 500 degrees Celsius/hr, from about 500 degreesCelsius/hr to about 750 degrees Celsius/hr, or from about 750 degreesCelsius/hr to about 1000 degrees Celsius/hr. In one embodiment, the ramprate may be selected to minimize thermal shock to the grown crystallinecomposition 132. The cell, including the capsule and pressure medium,may be removed from the pressure vessel 210 and the capsule 100 may beremoved from the cell.

The solvent 130 may be removed by chilling the capsule to reduce thevapor pressure of the solvent below 1 bar, puncturing the capsule, andwarming to evaporate the solvent. In another embodiment, the capsule maybe punctured at or near room temperature, for example, by drilling asmall hole or cutting off a fill tube, and the solvent may escape into ahood or other ventilated space. The capsule may be cut open and thegrown crystal(s) removed. The crystal(s) may be washed by an appropriatewash, such as water, alcohol or other organic solvent, and by mineralacids to remove mineralizer.

In an alternative embodiment, a high quality seed crystal, free of tiltboundaries and with a dislocation density below about 10⁴ cm⁻², may beused as a substrate for deposition of a thick film of AlInGaN by anothercrystalline composition growth method. In one embodiment, the othercrystalline composition growth method includes hydride vapor phaseepitaxy (HVPE).

Characterization techniques, such as photoluminescence, may indicate thequality of the crystalline composition. Photoluminescence may occur atthe band edge at room temperature for gallium nitride.

The crystalline composition may be processed and sliced into one or morewafers, lapped, polished, and/or chemically polished. Methods forslicing include sawing with a wire saw, a multi-wire saw, or an annularsaw. Lapping and polishing may be performed with a slurry containing oneor more diamond, silicon carbide, alumina, or other hard particles.Polishing may leave lattice damage in the gallium nitride wafer that maybe removed by a number of methods, including chemical mechanicalpolishing, dry etching by reactive ion etching (RIE), high densityinductively-coupled plasma (ICP) plasma etching, electron cyclotronresonance (ECR) plasma etching, and chemically assisted ion beam etching(CAIBE).

In another embodiment, the damaged GaN surface layer resulting fromwafer polishing is oxidized and the oxide layer is then removed.Oxidation of the damaged surface layer may be carried out using at leastone of a wet or dry thermal process, a plasma-assisted process, a UVlight-enhanced process, photoelectrochemical oxidation, or a combinationthereof. Wet oxidation may be performed by exposing the surface layer toan oxidizing agent, such as nitric acid, aqua regia, a mixture ofsulfuric acid and nitric acid, perchloric acid, or a persulfate,chromate, dichromate, or permanganate salt at a temperature between 0°C. and 200° C. Dry oxidation may be performed by heating in anoxygen-containing ambient to a temperature between about 200° C. andabout 900° C. or exposure to an oxygen-containing plasma. Wet oxidationmay also be performed by exposure of the surface to ultraviolet lightwhile in contact with a basic solution, optionally also containing anoxidizing agent or subjected to a positive electrical bias. The basicsolution may comprise alkali hydroxide at a concentration of 0.005 M to0.2 M. The ultraviolet light may contain radiation at wavelengths below365 nm and may be produced by an unfiltered mercury arc lamp. The oxidelayer may be removed by thermal annealing, for example, to a temperaturebetween about 600° C. and about 1200° C. in vacuum or in a nitrogen- orammonia-containing ambient, or wet chemical etching, for example, inalkali hydroxide or in concentrated acid. The oxidation andoxide-removal processes may be performed for durations between about10⁻⁵ sec and about 60 min and may be applied repetitively. In oneembodiment, both the oxidation and oxide removal steps are performed ina reactor suitable for performing epitaxial growth by metallorganicchemical vapor deposition or molecular beam epitaxy.

In another embodiment, the damaged GaN layer is annealed out, etchedaway, or thermally desorbed by heating to a temperature of 800-1500° C.,for a time between 0.001 sec and 10 hours in a nitrogen- and/orhydrogen-containing ambient, such as nitrogen, hydrogen, ammonia,hydrazine, hydrazoic acid, a nitrogen plasma, or plasma-dissociatedammonia. In one embodiment, the annealing process is performed in areactor suitable for performing epitaxial growth by metallorganicchemical vapor deposition or molecular beam epitaxy.

In another embodiment, lattice damage is removed by chemical mechanicalpolishing. The chemical mechanical polishing slurry may contain one ormore of abrasive particles, pH modifiers, viscosity modifiers,dispersion agents, chelating agents, non-ionic surfactants, polymericsurfactants, amine or imine surfactants, tetra-alkyl ammonium compounds,fluoride salts, ammonium salts, and oxidizing agents. The abrasiveparticles may comprise one or more of silicon oxide, aluminum oxide,cerium oxide, chromium oxide, zirconium oxide, titanium oxide, halfniumoxide, molybdenum oxide, tungsten oxide, copper oxide, iron oxide,nickel oxide, manganese oxide, tin oxide, rare earth oxide, titaniumdiboride, titanium carbide, tungsten carbide, cubic boron nitride,silicon nitride, aluminum nitride, titanium nitride, glass powder withcomposition comprising at least one of SiO₂ or Al₂O₃, or metallicparticles, and may have a diameter between about 10 nm and about 300 nm.In one embodiment, pH modifiers are added so that the pH of the slurryis between 0.5 and 4. In another embodiment the pH of the slurry isbetween 4 and 8, and in yet another the pH is between 8 and 13.5. Theoxidizing agent may comprise at least one of hydrogen peroxide, nitrocompounds, and potassium ferricyanide. The chelating agent may compriseethylenediaminetetraacetic acid or the like.

The polished wafer may have an RMS surface roughness below about 1 nmover a lateral area of at least 10×10 mm². The surface roughness may bebelow 0.5 nm over a lateral area of at least 10×10 mm². The wafer orsubstrate has a thickness in a range of from about 0.01 and 10 mm, mostin a range of from about 0.05 and 5 mm. The surface of the galliumnitride wafer may be flat to less than 1 micron. The front and backsurfaces of the gallium nitride wafer may be parallel to better than 1°.In one embodiment, the crystallographic orientation of the front of thegallium nitride wafer may be within about 10° of one of the (0001)orientation, the (0001) orientation, the (1010) orientation, the (1120)orientation, and the (1011) orientation. In one embodiment, theorientation of the front of the gallium nitride wafer may be withinabout 5° of one of these orientations.

The gallium nitride may be ground to a round or square shape, with oneor more additional flats to indicate the crystallographic orientation.In one embodiment 700, the edge of the gallium nitride wafer may besimply ground, as shown schematically in FIG. 7(a). However, theinventors have found that gallium nitride crystalline compositions orwafers crack easily, and a gallium nitride wafer with a simply groundedge may be particularly susceptible to chipping and cracking. In oneembodiment 710, a chamfer may be ground on at least one of the front andback surfaces, as shown schematically in FIG. 7(b). The chamfer may beground on the edge of the wafer using apparatus that may be well knownin the art. The depth of the chamfer (dimension a in FIG. 7(b)) may bein a range of from about 10 micrometers and 0.2t (dimension t in FIG.7(b)), where t may be the thickness of the wafer. The width of thechamfer (dimension b in FIG. 7(b)) may be between a and 5a. If both thetop (side on which epitaxy may be performed) and the bottom of the wafermay be chamfered, the larger chamfer may be placed on the bottom. Aslight curvature may be present at the edges of the chamfered portionsrather than sharp edges. In addition to reducing the tendency for thewafer to be chipped or cracked during handling, the chamfer also reducesthe likelihood of crowning or poor morphology of epitaxially-grownAlInGaN near the periphery of the wafer. In one embodiment, the waferedge 720 may be rounded, as shown schematically in FIG. 7(c). The radiusof curvature of the top edge (dimension r₁ in FIG. 7(c)) of the wafermay be between 10 μm and 0.5t (dimension t in FIG. 7(c)), where t may bethe thickness of the wafer. The angle Θ between the inside edge of therounded portion and the top surface of the wafer may be less than 30degrees. The radius of curvature (dimension r₂ in FIG. 7(c)) of thebottom edge of the wafer may be greater than r₁, and also forms an anglewith the bottom surface of the wafer that may be less than 30 degrees.The thickness of the unrounded edge (dimension w in FIG. 7(c)) of thewafer may be zero and may be less than 0.5 t.

This crystalline composition gallium nitride crystal, and wafers formedtherefrom, may be useful as substrates for electronic and optoelectronicdevices.

The crystalline composition may be characterized by standard methods.For determining the dislocation density, Cathodoluminescence (CL) andetch pit density may be convenient. CL imaging may provide anon-destructive measure of dislocation density, and requires little orno sample preparation. Dislocations may be non-radiative recombinationcenters in gallium nitride, and therefore appear in CL as dark spots.One can simply measure the concentration of dark spots in CL images todetermine the dislocation density. A second test method may be etch pitdensity.

Both of these methods were applied to the gallium face of a sample ofcommercial-grade HVPE gallium nitride dislocation densities (dark-spotdensities or etch pit densities) of 1-2×10⁷ cm⁻² were obtained, inexcellent agreement with the values reported by others on similarmaterial and those values shown in FIG. 9.

The optical absorption and emission properties of the grown galliumnitride can be determined by optical absorption, scattering, andphotoluminescence spectroscopies. The electrical properties can bedetermined by Van der Pauw and Hall measurements, by mercury-probe CV,or by hot-probe techniques.

The crystalline composition may be sliced into one or more wafers bymethods that may be well known in the art. The gallium nitridecrystalline composition or wafer may be useful as a substrate forepitaxial Al_(x)In_(y)Ga_(1-x-y)N films where 0≤x≤1, 0≤y≤1 and 0≤x+y≤1,light emitting diodes, laser diodes, photodetectors, avalanchephotodiodes, transistors, diodes, and other optoelectronic andelectronic devices. Epitaxial gallium nitride or Al_(x)In_(y)Ga_(1-x-y)Nlayers, where 0≤x, y, x+y≤1, deposited on gallium nitride wafersfabricated from a bulk gallium nitride crystalline composition describedherein may be free of tilt boundaries and may have a dislocation densitybelow 10⁴ cm⁻². In one embodiment, the below 10³ cm⁻², and, even morepreferably, below 100 cm⁻².

Due to the substantial absence of tilt boundaries and the lowdislocation density of the substrate, the homoepitaxial light-emittingdevice may be free of tilt boundaries. In one embodiment, for a devicearea up to about 10⁴ μm², up to about 9×10⁴ μm², or up to 1 mm² thedevice may be free of threading dislocations.

The above described embodiments provide improved nucleation control byincluding an equilibration period in the temperature program, in whichthe temperature gradient may be reduced, or even set to be zero ornegative, with respect to the gradient during crystalline compositiongrowth, and by hanging the seed crystal within the growth chamber. Thecrystalline composition growth method may provide high quality, largearea gallium nitride crystals.

A gallium nitride crystalline composition formed by the above method wascharacterized using etch pit density measurements, photoluminescence,and optical absorption techniques. The crystalline composition formedmay be characterized by a dislocation density below 100 cm⁻¹, aphotoluminescence spectrum which peaks at a photon energy of in a rangeof from about 3.38 to about 3.41 eV at a crystalline compositiontemperature of 300°K, and has an optical absorption coefficient below 5cm⁻¹ for wavelengths between 700 nm (red) and 465 nm (blue).

A gallium nitride crystalline composition formed by the above method wascharacterized by infrared transmission spectroscopy and by Ramanspectroscopy. In contrast to gallium nitride grown by other methods, thegallium nitride grown by the method described herein had several sharpabsorption peaks in the range of 3050 to 3300 cm⁻¹, with a maximumabsorption near 3175 cm⁻¹, as shown in FIG. 8. The crystallinecomposition was annealed to 750° C. in high purity nitrogen for 30 minand the infrared spectrum was re-measured. The absorption peaks in therange of 3050 to 3300 cm⁻¹ were essentially unchanged, as shown in FIG.8, indicating a high stability of the species responsible for theabsorption peaks. Based on predictions of vibrational frequencies of3100-3470 cm⁻¹ for V_(Ga)H₁-V_(Ga)H₄ (which may overestimate the actualfrequencies by about 200 cm⁻¹) and the observation of infraredabsorption features at 3020-3050 cm⁻¹ and at 3140 cm⁻¹ inhydrogen-implanted gallium nitride [M. G. Weinstein et al., Appl. Phys.Lett. 72, 1703 (1998)], the absorption peaks between 3150 and 3200 cm⁻¹in samples according to embodiments correspond to V_(Ga)H₃ and V_(Ga)H₄,that the absorption peaks observed between 3000 and 3150 cm⁻¹ in boththe crystalline composition and in the hydrogen-implanted galliumnitride correspond to V_(Ga)H₁ and V_(Ga)H₂, and that other minor peaksmay be associated with the presence of other impurities or defects. Thepresence of an infrared absorption feature near 3175 cm⁻¹ in galliumnitride crystalline composition grown by the method described hereinindicates passivation of gallium vacancies, and the persistence of theinfrared feature upon high temperature annealing indicates that thispassivation may be quite stable. Depending on the concentration ofhydrogenated gallium vacancies in the gallium nitride crystal, theabsorbance per unit thickness of the 3175 cm⁻¹ peak may lie in a rangeof from about 0.01 and 200 cm⁻¹.

Additional evidence for the passivation of point defects in a galliumnitride crystalline composition grown by the method described herein maybe obtained by Raman spectroscopy. A total of five peaks may be observedin two configurations between 400 and 800 cm⁻¹. The peaks, with therespective assignments given in brackets, were observed at 530 cm⁻¹(A₁(TO)], 558 cm⁻¹ [E₁ (TO)], 569 cm⁻¹ [E₂ (high)], 734 cm⁻¹ [A₁(LO)],and 742 cm⁻¹ [E₁(LO)]. These values may be all within a few cm⁻¹ ofaccepted values for pure gallium nitride reported in the literature.Significantly, a broad peak associated with phonon-plasmon coupling wasnot observed. The observation of unshifted LO modes and the absence of aphonon-plasmon mode indicates a carrier concentration below 10¹⁷ cm⁻³,based on Raman measurements reported in the literature on galliumnitride with carrier concentrations between 10¹⁶ cm⁻³ and 10²⁰ cm⁻³. Thetotal impurity concentration in this crystalline composition was above10¹⁹ cm⁻³. The drastic reduction in carrier concentration relative tothe impurity concentration indicates a high degree of compensation, mostlikely due to hydrogen.

The incorporated hydrogen may be benign or possibly even beneficial. Byway of contrast, typical or conventional gallium nitride growth methodsmay not provide passivation of gallium vacancies by hydrogenation, evenif hydrogen may be in the growth system. For example, infraredtransmission spectroscopy on 300-400 millimeters thick gallium nitridesamples grown by hydride vapor phase epitaxy (HVPE) revealed weakabsorption features near 2850 and 2915 cm⁻¹ associated with anotherdefect, but no absorption features between 3100 and 3500 cm⁻¹ that couldbe assigned to hydrogenated gallium vacancies were observed in the HVPEgallium nitride material.

Continuing with passivated gallium vacancies, the lattice structure andchemical and electrical properties of a crystalline composition maydiffer relative to a crystal without vacancies, with vacancies that arenot passivated, and over differing levels of passivation. In oneembodiment, the gallium-poor crystal may be formed, and then passivatedusing, for example, hydrogen interfusion at elevated temperature andpressure. In another embodiment, the vacancies are formed, andpassivated, in a non-gallium crystal. Control of the level of vacancies(for example, by control of the raw material type, quantity, orprocessing conditions) and of the level of passivation may allow fortailoring of the crystalline composition properties in a determinedmanner.

Within the visible spectrum, a gallium nitride boule may be transparentand colorless. The optical absorption coefficient for nominally undopedcrystalline composition may be less than 5 cm⁻¹ between 465 nm and 700nm. Doped crystalline composition may exhibit similarly low absorption,although some free carrier absorption may be introduced at high carrierconcentrations. Moreover, dopants, substitutional or interstitialimpurities, vacancy complexes, or other point defects may introducenarrow peaks of higher absorption within the visible range. Such pointdefect-related narrow absorption peaks may not, however, significantlyreduce the transparency of the crystalline composition in the visible,such as in the backside extraction of emitted light.

In the case where a gallium nitride boule may be grown using at leastone of HX, NH₄, GaX₃, (where X is halogen), or other compoundsobtainable by reaction of Ga, gallium nitride, NH₃, and HF, asmineralizer, the gallium nitride may contain at least about 0.04 ppmfluorine, and in a range of from about 0.04 and 1 ppm fluorine. Bycontrast, gallium nitride crystalline composition grown withfluorine-free mineralizers contain less than 0.02 ppm fluorine. As withthe case of incorporated hydrogen, the incorporated fluorine may bebelieved to be benign or possibly even beneficial. Bond lengths tofluorine in molecules or solids may be only slightly larger than thecorresponding bonds to hydrogen, so that fluorine may play a similarrole passivating defects.

After the gallium nitride crystalline composition forms, the crystallinecomposition or boule may be processed and sliced into one or morewafers, lapped, polished, and chemically polished. The wafer orsubstrate has a thickness in a range of from about 0.01 millimeters toabout 0.05 millimeters, from about 0.05 millimeters to about 5millimeters, or from about 5 millimeters to about 10 millimeters, andmay be useful as a substrate for the device fabrication. A suitablewafer may include n-type gallium nitride, with an electrical resistivityless than about 100 Ω-cm. In one embodiment, the wafer may have anelectrical resistivity less than about 10 Ω-cm, in a range of from about10 Ω-cm to about 1 Ω-cm, or less than about 1 Ω-cm. In one embodiment,the wafer includes p-type gallium nitride, and in still anotherembodiment the wafer includes semi-insulating gallium nitride. Thesubstrate may be polished to a mirror finish using mechanical-polishingtechniques that may be known in the art. Subsurface damage may remainafter the polishing process. This damage may be removed by severalmethods that may be known in the art, including chemically assisted ionbeam etching, reactive ion etching, chemo-mechanical polishing, andphotoelectrochemical or wet chemical etching.

The residual damage may be removed by heating the wafer to a temperaturein a range of from about 700 degrees Celsius to about 1500 degreesCelsius in a nitrogen-containing atmosphere, such as, for example, N₂gas or ammonia, at a partial pressure in a range of from about 10⁻⁸ mbarto about 20,000 bar. The substrate has a thickness in a range of fromabout 0.01 millimeters and 0.05 mm, in a range of from about 0.05millimeters to about 5 millimeters, or from about 5 millimeters to about10 millimeters.

A gallium nitride crystalline composition may be provided that is atleast about 2 millimeters in at least one dimension x or y, with adislocation density of less than about 10⁴ cm⁻¹, and having no tiltboundaries. In one embodiment, at least one dimension x or y is in arange of from about 2 mm to about 2.75 mm, from about 2.75 mm to about 3mm, from about 3 mm to about 5 mm, from about 5 mm to about 1centimeter, from about 1 centimeter to about 2 centimeter, from about 2centimeters to about 7.5 centimeters, from about 7.5 centimeters toabout 10 centimeters, or greater than about 10 centimeters. A galliumnitride crystalline composition may be at least about 2 millimeters inat least one dimension x or y, and free of tilt boundaries, and may havea photoluminescence spectrum which peaks at a photon energy of in arange of from about 3.38 to about 3.41 eV at a crystalline compositiontemperature of 300 K.

In accordance with another aspect of the invention, there may beprovided a method of forming a gallium nitride single crystal. Themethod includes (a) providing a nucleation center in a first region of achamber; (b) providing a gallium nitride source material in a secondregion of the chamber; (c) providing a gallium nitride solvent in thechamber; (d) pressurizing the chamber; (e) generating and holding afirst temperature distribution such that the solvent may besupersaturated in the first region of the chamber and such that theremay be a first temperature gradient between the nucleation center andthe gallium nitride source material such that gallium nitridecrystalline composition grows on the nucleation center; and (f)generating a second temperature distribution in the chamber such thatthe solvent may be supersaturated in the first region of the chamber andsuch that there may be a second temperature gradient between thenucleation center and the gallium nitride source material such thatgallium nitride crystalline composition grows on the nucleation center,wherein the second temperature gradient may be larger in magnitude thanthe first temperature gradient and the crystalline composition growthrate may be greater for the second temperature distribution than for thefirst temperature distribution.

In accordance with another aspect of the invention, a method of forminga gallium nitride single crystal or quasi-single crystal is provided.The method includes (a) providing a nucleation center in a first regionof a chamber having a first end; (b) providing a gallium nitride sourcematerial in a second region of the chamber having a second end; (c)providing a gallium nitride solvent in the chamber; (d) pressurizing thechamber to a pressure in a range of from about 5 kbar to 10 kbar, fromabout 10 kbar to about 25 kbar, or from about 25 millimeters to about 80kbar; (e) generating and holding a first temperature distribution havingan average temperature in a range of from about 550 degrees Celsius toabout 1200 degrees Celsius such that the solvent may be supersaturatedin the first region of the chamber and such that there may be a firsttemperature gradient between the first end and the second end such thatgallium nitride crystalline composition grows on the nucleation center;and (f) generating a second temperature distribution in the chamberhaving an average temperature in a range of from about 550 degreesCelsius to about 1200 degrees Celsius such that the solvent may besupersaturated in the first region of the chamber and such that theremay be a second temperature gradient between the first end and thesecond end such that gallium nitride crystalline composition grows onthe nucleation center, wherein the second temperature gradient may belarger in magnitude than the first temperature gradient and thecrystalline composition growth rate may be greater for the secondtemperature distribution than for the first temperature distribution.

In accordance with another aspect of the invention, a method may be usedto form a gallium nitride crystalline composition. The method includes(a) providing a nucleation center in a first region of a chamber havinga first end; (b) providing a gallium nitride source material in a secondregion of the chamber having a second end; (c) providing a galliumnitride solvent in the chamber; (d) pressurizing the chamber; (e)generating and holding a first temperature distribution such that theremay be a first temperature gradient between the first end and the secondend; and (f) generating a second temperature distribution in the chambersuch that the solvent may be supersaturated in the first region of thechamber and such that there may be a second temperature gradient betweenthe first end and the second end such that gallium nitride crystallinecomposition grows on the nucleation center, wherein the firsttemperature gradient may be zero or opposite in sign from the secondtemperature gradient. The crystalline composition may be a singlecrystal.

EXAMPLES

The following examples illustrate methods and embodiments in accordancewith the invention and do not limit the claims. Unless specifiedotherwise, all ingredients are commercially available from such commonchemical suppliers as Alpha Aesar, Inc. (Ward Hill, Mass.),Sigma-Aldrich Company (St. Louis, Mo.), and the like. The followingComparative Examples (Comparative Examples 1-3) are provided forcomparison to the Examples (Examples 1-4).

Comparative Example 1

0.1 grams of NH₄F mineralizer is placed in the bottom of an about 1.25centimeters diameter silver capsule. A baffle with 5.0 percent open areais placed in the middle portion of the capsule, and 0.31 grams ofpolycrystalline gallium nitride source material is placed in the upperhalf of the capsule. The capsule is enclosed within a filler/sealingassembly together with a 1.25 centimeters diameter steel ring. Thecapsule and filler/sealing assembly are transferred to a gas manifoldand filled with 0.99 grams of ammonia. Next, a plug is inserted into theopen top end of the capsule, such that a cold weld is formed between thesilver capsule and silver plug and the steel ring surrounded the plugand provided reinforcement. The capsule is removed from thefiller/sealing assembly and inserted in a zero stroke high-pressure hightemperature (HPHT) apparatus. The cell is heated to approximately 700degrees Celsius and held at this temperature for 55 hours, with atemperature gradient of approximately 85 degrees Celsius. The cell iscooled and removed from the press.

Upon opening the capsule after venting of the ammonia, numerousspontaneously-nucleated crystalline composition are observed at thebottom of the capsule. One crystalline composition approximately 0.36millimeters in diameter is selected at random and etched in 10 percenthydrochloric acid (HCl) in Argon at 625 degrees Celsius for 30 minutes.No etch pits are observed. The area of the exposed c-face isapproximately 5.3×10⁻⁴ cm², indicating that the etch pit density is lessthan (1/(5.3×10⁻⁴ cm²)) or 1900 cm⁻². By contrast, the etching treatmentis applied to a 200 μm-thick piece of gallium nitride grown byhydride/halide vapor phase epitaxy (HVPE), and an etch pit density of2×10⁷ cm⁻² is observed on the gallium face. The observed etch pitdensity of the HVPE-grown sample is in good agreement with FIG. 9 formaterial that is grown to a thickness of about 300 micrometers beforebeing lapped and polished.

Comparative Example 2

Three seeds, weighing 3 mg to 4 mg each, are placed in the bottom of aabout 1.25 centimeters diameter silver capsule along with 0.10 grams ofNH₄F mineralizer. A baffle with 5.0 percent open area is placed in themiddle portion of the capsule, and 0.34 grams of polycrystalline galliumnitride source material is placed in the upper half of the capsule. Thecapsule is enclosed within a filler/sealing assembly together with a0.675 inch diameter steel ring. The capsule and filler/sealing assemblyare transferred to the gas manifold and filled with 1.03 grams ofammonia. Next, the plug is inserted into the open top end of thecapsule, such that a cold weld is formed between the silver capsule andsilver plug and the steel ring surrounded the plug and providedreinforcement. The capsule is removed from the filler/sealing assemblyand inserted in a zero stroke HPHT apparatus.

The cell is heated at about 15 degrees Celsius/min to approximately 500degrees Celsius, at 0.046 degrees Celsius/min to 700 degrees Celsius,and held at the latter temperature for 6 hours, with a temperaturegradient of approximately 28 degrees Celsius. The cell is cooled andremoved from the press. Upon opening the capsule after venting of theammonia, numerous spontaneously-nucleated crystalline composition areobserved at the bottom of the capsule and, despite the very slow heatingrate, very little growth on the seeds occurred, relative to growth onspontaneously-nucleated crystals.

Comparative Example 3

A gallium nitride seed, weighing 10.4 mg, is placed in the bottom of aabout 1.25 centimeters diameter silver capsule along with 0.04 grams ofNH₄F mineralizer. A baffle with 5.0 percent open area is placed in themiddle portion of the capsule, and 0.74 grams of polycrystalline galliumnitride source material is placed in the upper half of the capsule. Thecapsule is enclosed within a filler/sealing assembly together with a0.675 inch diameter steel ring. The capsule and filler/sealing assemblyare transferred to the gas manifold and filled with 1.14 grams ofammonia. Next, the plug is inserted into the open top end of thecapsule, such that a cold weld is formed between the silver capsule andsilver plug and the steel ring surrounded the plug and providedreinforcement. The capsule is removed from the filler/sealing assemblyand inserted in a zero stroke HPHT apparatus. The cell is heated atabout 15 degrees Celsius/min to approximately 500 degrees Celsius, at0.05 degrees Celsius/min to 680 degrees Celsius, and held at the lattertemperature for 53 hours, with a temperature gradient of approximately70 degrees Celsius. The cell is cooled and removed from the press. Uponopening the capsule after venting of the ammonia, numerousspontaneously-nucleated crystalline compositions are observed at thebottom of the capsule despite the very slow heating rate. The seed growsto a weight of 41.7 mg and a diameter of about 2 mm. However, the weightof spontaneously-nucleated crystalline compositions is more than 10times the weight increase of the seed.

Example 1

A small hole is drilled by a high-power laser through a gallium nitrideseed crystal weighing 19.7 mg. The seed is hung by a 0.13-mm silver wirefrom a silver baffle with a 35 percent open area and placed in the lowerhalf of a about 1.25 centimeters diameter silver capsule along with 0.10grams of NH₄F mineralizer. 0.74 grams of polycrystalline gallium nitridesource material is placed in the upper half of the capsule. The capsuleis enclosed within a filler/sealing assembly together with a 0.583 inchdiameter steel ring. The capsule and filler/sealing assembly aretransferred to a gas manifold and filled with 0.99 grams of ammonia.Next, the plug is inserted into the open top end of the capsule, suchthat a cold weld is formed between the silver capsule and silver plugand the steel ring surrounded the plug and provided reinforcement. Thecapsule is removed from the filler/sealing assembly and inserted in azero stroke HPHT apparatus.

The cell is heated at a rate of about 11 degrees Celsius/min until thetemperature of the bottom of the capsule is approximately 700 degreesCelsius and the temperature of the top half of the capsule isapproximately 660 degrees Celsius, as measured by type K thermocouples.The current through the top half of the heater is increased until thetemperature gradient ΔT decreased to zero. After holding at ΔT=0 for 1hour, the temperature of the top half of the capsule is decreased at 5degrees Celsius/hr until ΔT □□ increased to approximately 35 degreesCelsius, and the temperatures are held at these values for 78 hr. Thecell is cooled and removed from the press. Upon opening the capsuleafter venting of the ammonia, the seed weight is observed to haveincreased to 33.4 mg.

The crystalline composition is characterized by photoluminescence, usinga 266 nm excitation (frequency-quadrupled YAG). The spectra at severaltemperatures are shown in FIG. 3. Specifically the crystallinecomposition sample is characterized by photoluminescence at temperaturesof 5 K, 20 K, 77 K and 300 K. At all temperatures in the range of 5K-300K, the luminescence peak occurs between 3.38 and 3.45 eV.

Example 2

A gallium nitride seed crystal weighing 12.6 mg, obtained from aprevious run, is hung through a laser-drilled hole by a 0.13-mm silverwire from a silver baffle with a 35 percent open area and placed in thelower half of a about 1.25 centimeters diameter silver capsule. 0.10grams of NH₄F mineralizer and 1.09 grams of polycrystalline galliumnitride source material are placed in the upper half of the capsule. Thecapsule is enclosed within a filler/sealing assembly together with a0.583 inch diameter steel ring. The capsule and filler/sealing assemblyare transferred to the gas manifold and filled with 0.95 grams ofammonia. Next, the plug is inserted into the open top end of thecapsule, such that a cold weld is formed between the silver capsule andsilver plug and the steel ring surrounded the plug and providedreinforcement. The capsule is removed from the filler/sealing assemblyand inserted in a zero stroke HPHT apparatus. The cell is heated at arate of about 11 degrees Celsius/min until the temperature of the bottomof the capsule is approximately 700 degrees Celsius and the temperatureof the top half of the capsule is approximately 640 degrees Celsius, asmeasured by type K thermocouples. The current through the top half ofthe heater is increased until the temperature gradient ΔT decreased tozero. After holding at ΔT=0 for 1 hour, the temperature of the top halfof the capsule is decreased at 5 degrees Celsius/hr until ΔT increasedto approximately 50 degrees Celsius, and the temperatures are held atthese values for 98 hr. The cell is cooled and removed from the press.

Upon opening the capsule after venting of the ammonia, the seed hasgrown to a weight of 24.3 mg. The crystalline composition is etched in10 percent HCl in Ar at 625 degrees Celsius for 30 min. Some etch pitsare observed on the c-face above the region of the seed, with an etchpit density of about 10⁶ cm⁻². However, the areas that grow laterallywith respect to the seed are free of etch pits. The area of newlylaterally-grown gallium nitride is approximately 3.2×10⁻² cm²,indicating that the etch pit density is less than (1/3.2×10⁻² cm²) or 32cm⁻².

Example 3

Two gallium nitride seeds, weighing 48.4 mg and 36.6 mg and obtainedfrom a previous run, are hung through laser-drilled holes by a 0.13-mmsilver wire from a silver baffle with a 35 percent open area and placedin the lower half of a about 1.25 centimeters diameter silver capsule.0.10 grams of NH₄F mineralizer and 1.03 grams of polycrystalline galliumnitride source material are placed in the upper half of the capsule. Thecapsule is enclosed within a filler/sealing assembly together with a0.583 inch diameter steel ring. The capsule and filler/sealing assemblyare transferred to the gas manifold and filled with 1.08 grams ofammonia. Next, the plug is inserted into the open top end of thecapsule, such that a cold weld is formed between the silver capsule andsilver plug and the steel ring surrounded the plug and providedreinforcement. The capsule is removed from the filler/sealing assemblyand inserted in a zero stroke HPHT apparatus. The cell is heated atabout 11 degrees Celsius/min until the temperature of the bottom of thecapsule is approximately 700 degrees Celsius and the temperature of thetop half of the capsule is approximately 642 degrees Celsius, asmeasured by type K thermocouples. The current through the top half ofthe heater is increased until the temperature gradient ΔT decreased tozero. After holding at ΔT=0 for 1 hour, the temperature of the top halfof the capsule is decreased at 5 degrees Celsius/hr until ΔT increasedto approximately 30 degrees Celsius, and the temperatures are held atthese values for 100 hr. The cell is cooled and removed from the press.

Upon opening the capsule after venting of the ammonia, the seeds hadgrown to a weight of 219.8 mg. A piece broke off from the smaller of thetwo crystalline compositions and is selected for analysis. An opticaltransmission spectrum of the crystalline composition is measured using aCary 500i spectrometer. The transmission is greater than 60 percent forwavelengths ranging from red (700 cm⁻¹) to blue (465 cm⁻¹). Based on theindex of refraction for gallium nitride [G Yu et al., Applied PhysicsLetters 70, 3209 (1997), which is hereby incorporated by reference] andthe thickness of the crystal, 0.206 mm, the optical absorptioncoefficient is less than 5 cm⁻¹ over the same wavelength range. Thecrystalline composition is determined to have n-type electricalconductivity by means of a hot-point probe measurement. The crystallinecomposition is etched in 10 percent HCl in Ar at 625 degrees Celsius for30 min. The entire crystalline composition is free of etch pits. Thearea of the c-face of the crystalline composition is approximately4.4×10⁻² cm², indicating that the etch pit density is less than(1/4.4×10⁻² cm²) or 23 cm⁻².

Example 4

A gallium nitride seed weighing 25.3 mg, obtained from a previous run,is hung through a laser-drilled hole by a 0.13-mm silver wire from asilver baffle with a 35 percent open area and placed in the lower halfof a about 1.25 centimeters diameter silver capsule. 0.10 grams of NH₄Fmineralizer and 0.98 grams of polycrystalline gallium nitride sourcematerial are placed in the upper half of the capsule. The capsule isenclosed within a filler/sealing assembly together with a 0.583 inchdiameter steel ring. The capsule and filler/sealing assembly aretransferred to the gas manifold and filled with 1.07 grams of ammonia.Next, the plug is inserted into the open top end of the capsule, suchthat a cold weld is formed between the silver capsule and silver plugand the steel ring surrounded the plug and provided reinforcement. Thecapsule is removed from the filler/sealing assembly and inserted in azero stroke HPHT apparatus. The cell is heated at about 11 degreesCelsius/min until the temperature of the bottom of the capsule isapproximately 700 degrees Celsius and the temperature of the top half ofthe capsule is approximately 648 degrees Celsius, as measured by type Kthermocouples. The current through the top half of the heater isincreased until the temperature gradient ΔT decreased to 3 degreesCelsius. After holding at ΔT=3 degrees Celsius for 1 hour, thetemperature of the top half of the capsule is decreased at 5 degreesCelsius/hr until ΔT□ increased to approximately 30 degrees Celsius,decreased further at 2.5 degrees Celsius/hr until ΔT increased toapproximately 60 degrees Celsius and the temperatures are held at thesevalues for 20 hr. The cell is cooled and removed from the press.

Upon opening the capsule after venting of the ammonia, the seed hadgrown to a weight of 40.2 mg. The crystalline composition is etched in50 percent HNO₃ for 30 min. A row of etch pits is observed on the c-faceabove the interface between the seed and new, laterally-grown material.However, the remaining areas of newly-grown gallium nitride are free ofetch pits. The area of pit-free newly grown gallium nitride isapproximately 6.9×10⁻² cm², indicating that the etch pit density is lessthan (1/6.9×10⁻² cm²) or 14 cm⁻².

Example 5

A gallium nitride seed weighing 13.5 mg, grown by HVPE, is hung througha laser-drilled hole by a 0.13-mm silver wire from a silver baffle witha 35 percent open area and placed in the lower half of a about 1.25centimeters diameter silver capsule. 0.10 grams of NH₄F mineralizer,0.031 grams of CoF₂, and 0.304 grams of polycrystalline gallium nitridesource material are placed in the upper half of the capsule. The capsuleis enclosed within a filler/sealing assembly together with a 0.583 inchdiameter steel ring. The capsule and filler/sealing assembly aretransferred to the gas manifold and filled with 1.01 grams of ammonia.Next, the plug is inserted into the open top end of the capsule, suchthat a cold weld is formed between the silver capsule and silver plugand the steel ring surrounded the plug and provided reinforcement. Thecapsule is removed from the filler/sealing assembly and inserted in azero stroke HPHT apparatus. The cell is heated at about 11 degreesCelsius/min until the temperature of the bottom of the capsule isapproximately 700 degrees Celsius and the temperature of the top half ofthe capsule is approximately 635 degrees Celsius, as measured by type Kthermocouples, and the temperatures are held at these values for 10 hr.The cell is cooled and removed from the press.

Upon opening the capsule after venting of the ammonia, the seed weightis 10.3 mg, but had become thicker (0.7 millimeters thick) and isessentially black; e.g., considerably darker in color than nominallyundoped crystals. Seed crystals used with NH₄F as a mineralizer undergoetching before the onset of crystalline composition growth. Afterwashing, the Co-doped gallium nitride crystalline composition issandwiched between two pieces of Indium foil which had been wet with aliquid Ga—In alloy with an electrode area of approximately 0.02 cm². Theelectrical resistance across the crystalline composition is found to beapproximately 1,050 MΩ at room temperature, corresponding to aresistivity of about 3×10⁸ Ω-cm. Gallium nitride with a resistivitygreater than about 10⁵ Ω-cm is semi-insulating. The crystallinecomposition is placed in a photoluminescence apparatus and illuminatedwith a 266-nm nitrogen laser. No photoluminescence is observable. Theratio of the intensity of near-band-edge photoluminescence from theblack gallium nitride crystalline composition to that of anear-transparent, nominally undoped gallium nitride crystallinecomposition is less than 0.1 percent.

Example 6

A gallium nitride seed grown by HVPE is hung through a laser-drilledhole by a 0.13-mm silver wire from a silver baffle with a 10 percentopen area and placed in the lower half of a about 1.25 centimetersdiameter silver capsule. 0.10 grams of NH₄F mineralizer, 0.087 grams ofFe_(x)N and 0.305 grams of polycrystalline gallium nitride sourcematerial are placed in the upper half of the capsule. The capsule isenclosed within a filler/sealing assembly together with a 0.583 inchdiameter steel ring. The capsule and filler/sealing assembly aretransferred to the gas manifold and filled with 1.12 grams of ammonia.Next, the plug is inserted into the open top end of the capsule, suchthat a cold weld is formed between the silver capsule and silver plugand the steel ring surrounded the plug and provided reinforcement. Thecapsule is removed from the filler/sealing assembly and inserted in azero stroke HPHT apparatus. The cell is heated at about 11 degreesCelsius/min until the temperature of the bottom of the capsule isapproximately 700 degrees Celsius and the temperature of the top half ofthe capsule is approximately 630 degrees Celsius, as measured by type Kthermocouples, and the temperatures are held at these values for 10 hr.The cell is cooled and removed from the press.

Upon opening the capsule after venting of the ammonia, the seed hadgrown to a thickness of 170 micrometers (μm) and had a reddish/ambercolor. After washing, the Fe-doped gallium nitride crystallinecomposition is sandwiched between two pieces of Indium foil which hadbeen wet with a liquid Ga—In alloy with an electrode area ofapproximately 0.02 cm². The electrical resistance is over 32 MΩ□ at roomtemperature, corresponding to a resistivity of over 3×10⁷ Ω-cm. Galliumnitride with a resistivity greater than about 10⁵ Ω-cm is considered assemi-insulating.

Example 7

A gallium nitride seed weighing 14.3 mg, grown by HVPE, is hung througha laser-drilled hole by a 0.13-mm silver wire from a silver baffle witha 35 percent open area and placed in the lower half of a about 1.25centimeters diameter silver capsule. 0.10 grams of NH₄F mineralizer,0.026 grams of Mn_(x)N and 1.008 grams of polycrystalline galliumnitride source material are placed in the upper half of the capsule. Thecapsule is enclosed within a filler/sealing assembly together with a0.583 inch diameter steel ring. The capsule and filler/sealing assemblyare transferred to the gas manifold and filled with 1.04 grams ofammonia. Next, the plug is inserted into the open top end of thecapsule, such that a cold weld is formed between the silver capsule andsilver plug and the steel ring surrounded the plug and providedreinforcement. The capsule is removed from the filler/sealing assemblyand inserted in a zero stroke HPHT apparatus. The cell is heated atabout 11 degrees Celsius/min until the temperature of the bottom of thecapsule is approximately 700 degrees Celsius and the temperature of thetop half of the capsule is approximately 650 degrees Celsius, asmeasured by type K thermocouples, and the temperatures are held at thesevalues for 60 hr. The cell is cooled and removed from the press.

Upon opening the capsule after venting of the ammonia, the seed hadgrown to a weight of 53.4 mg and is 350 micrometers thick and showed anorange color. Susceptibility measurements demonstrated that the Mn-dopedgallium nitride crystalline compositions are paramagnetic.

Example 8

0.100 g, 0.200 g, or 0.500 grams of NH₄F is added to three separateabout 1.25 centimeters silver capsules. Also added to each capsule are0.36 grams of polycrystalline gallium nitride and 0.9-1.0 grams ofammonia, using the filler/sealing assembly. The concentrations of NH₄Fmineralizer, expressed as a mole ratio with respect to ammonia, are 5.4percent, 9.3 percent, and 23.7 percent, respectively, in the threecapsules. The sealed capsules are placed in a cell in a zero-strokehigh-pressure apparatus and heated to 700 degrees Celsius, held at thistemperature for 8 hours, and cooled. Gallium nitride crystallinecompositions grew in all three capsules. Also present in each capsuleare crystalline compositions comprising GaF₃(NH₃)₂ and (NH₄)₃GaF₆. Theweights of the Ga-containing complexes are 0.12 g, 0.25 g, and 0.65 g,respectively, in the three capsules, indicating that the concentrationof dissolved Ga-containing species is approximately proportional to theinitial mineralizer concentration. The weights of undissolvedpolycrystalline gallium nitride in the three capsules are 0.29 g, 0.23g, and 0.03 g, respectively, indicating that higher concentrations ofmineralizer enabled more rapid dissolution and transport of galliumnitride.

Example 9

A hole 2 millimeters in diameter is laser-cut in the center of a 1-cmsquare gallium nitride seed crystal. The seed crystal is hung from a 25percent open-area baffle and placed inside a 1.1 inch diameter silvercapsule. 1.000 grams of NH₄F and 15.276 grams of polycrystalline galliumnitride are added to a 1.1 inch diameter silver capsule inside a glovebox, a lid with a 0.12 inch diameter fill tube is welded to the top ofthe capsule. The fill tube is attached to a gas manifold without any airexposure to the contents and the capsule is evacuated, filled with 8.44grams of NH₃. The fill tube is welded shut. The capsule is placed in acell in a zero-stroke high-pressure apparatus. The cell is heated atabout 11 degrees Celsius/min until the temperature of the bottom of thecapsule is approximately 700 degrees Celsius and the temperature of thetop half of the capsule is approximately 650 degrees Celsius, asmeasured by type K thermocouples. The current through the top half ofthe heater is increased until the temperature gradient ΔT decreased tozero. After holding at ΔT=0 for 1 hour, the temperature of the top halfof the capsule is decreased at 5° C./hr until ΔT increased toapproximately 30 degrees Celsius, and the temperatures are held at thesevalues for 100 hr. The cell is cooled and removed from the press.

Upon opening the capsule after venting of the ammonia, the seed is foundto have grown laterally to about 11.7×16.0 millimeters and filled in thehole in the center. The crystal, shown in FIG. 11, comprised essentiallydislocation-free material over the hole and at its periphery, althoughboundaries at the position where laterally-grown gallium nitridecoalesced over the seed are visible. The growth rate in the m directionis about 17 μm/hr and the growth rate in the a-direction is about 60μm/hr, more than enough to fill the hole in the seed with high qualitymaterial.

Example 10

A 18×18×18 millimeters long triangle shape gallium nitride seed crystalabout 0.2 millimeters thick is hung from a 15 percent open-area baffleand placed inside a 1.1 inch diameter silver capsule. 0.998 grams ofGaF₃, 0.125 grams of NH₄F, and 10.118 grams of polycrystalline galliumnitride are added to the capsule inside a glove box, a lid with a 0.12inch diameter fill tube is welded to the top of the capsule. The filltube is attached to a gas manifold without any air exposure to thecontents and the capsule is evacuated, and filled with 9.07 grams ofNH₃. The fill tube is welded shut. The capsule is placed in a cell in azero-stroke high-pressure apparatus. The cell is heated until thetemperature of the bottom of the capsule is approximately 750 degreesCelsius and the temperature of the top half of the capsule isapproximately 700 degrees Celsius, as measured by type K thermocouples.The temperatures are held at these values for 54 hr. The cell is cooledand removed from the press.

Upon opening the capsule after venting of the ammonia, the seed hasgrown laterally to about 20×20×20 mm. The growth rate lateral to the caxis is approximately 37 μm/hr. The crystal, shown in FIG. 10, comprisedessentially dislocation-free material on the edge area. The crystallinecomposition as grown is transparent without any visible cracks,two-dimensional boundaries, or other defects.

Example 11

A 18×13×0.20 millimeters thick triangle shape gallium nitride seedcrystal is hung from a 25 percent open-area baffle and placed inside a1.1 inch diameter silver capsule. 1.0 grams of NH₄F and 14.655 grams ofpolycrystalline gallium nitride are added to the capsule inside a glovebox, a lid with a 0.12 inch diameter fill tube is welded to the top ofthe capsule. The fill tube is attached to a gas manifold without any airexposure to the contents and the capsule is evacuated, filled with 8.35grams of NH₃. The fill tube is welded shut. The capsule is placed in acell in a zero-stroke high-pressure apparatus. The cell is heated untilthe temperature of the bottom of the capsule is approximately 700degrees Celsius and the temperature of the top half of the capsule isapproximately 660 degrees Celsius, as measured by type K thermocouples.The temperatures are held at these values for 99 hr. The cell is cooledand removed from the press.

Upon opening the capsule after venting of the ammonia, the lateraldimensions of the seed remained the same, about 18×13 mm. Thecrystalline composition is wedge shaped, with the thickness ranging from0.50 millimeters on the end near the baffle to 2.36 millimeters on theend near the bottom of the capsule. The growth rate is 5 microns/hralong the C (0001) direction on the thin end and 22 microns/hr on thethick end. The crystalline composition is dark green but transparentwithout any visible cracks, two-dimensional boundaries, or otherdefects.

Example 12

A 1×1 cm² size gallium nitride seed, 880 μm thick, is hung from a 10percent open-area baffle and placed inside a 1.1 inch diameter silvercapsule. 1.147 grams of GaF₃ and 10.112 grams of polycrystalline galliumnitride are added to the capsule inside a glove box, a lid with a 0.12inch diameter fill tube is welded to the top of the capsule. The filltube is attached to a gas manifold without any air exposure to thecontents and the capsule is evacuated, and filled with 8.35 grams ofNH₃. The fill tube is welded shut. The capsule is placed in a cell in azero-stroke high-pressure apparatus. The cell is heated until thetemperature of the bottom of the capsule is approximately 750 degreesCelsius and the temperature of the top half of the capsule isapproximately 705 degrees Celsius, as measured by type K thermocouples.The temperatures are held at these values for 56.5 hours. The cell iscooled and removed from the press.

Upon opening the capsule after venting of the ammonia, the seed hasincreased in thickness to 1520 mm, indicating a growth rate of 11.3microns/hr growth rate along the c (0001) direction.

Example 13

1.53 grams of NH₄F and 1.53 grams of polycrystalline gallium nitride areadded to about 1.25 centimeters silver capsule without any ammonia. Thesealed capsule is placed in a cell in a zero-stroke high-pressureapparatus and heated to 700 degrees Celsius, held at temperature for 13hours, and cooled. 0.42 of NH₃ gas formed by reaction of NH₄F withgallium nitride during the high temperature process is released when thecapsule is opened. A well-faceted, spontaneously-nucleated galliumnitride crystalline composition is recovered from the bottom of thecapsule. An equivalent of about 0.62 grams of NH₄F remains(1.53−37/17×0.42), which implies that gallium nitride growth occurs in40 mole percent NH₄F.

Example 14

A slot of 1.3×6.1 millimeters is laser-cut in the center of a 10×16×0.2millimeters HVPE gallium nitride crystal. The gallium nitride seedcrystal is hung from a 25 percent open-area baffle and placed inside a1.1 inch diameter silver capsule. 1.0 grams of NH₄F and 12.79 grams ofpolycrystalline gallium nitride are added to the capsule inside a glovebox, a lid with a 0.12 inch diameter fill tube is welded to the top ofthe capsule. The fill tube is attached to a gas manifold without any airexposure to the contents and the capsule is evacuated, filled with 8.17grams of NH₃. The fill tube is welded shut. The capsule is placed in acell in a zero-stroke high-pressure apparatus. The cell is heated untilthe temperature of the bottom of the capsule is approximately 700degrees Celsius and the temperature of the top half of the capsule isapproximately 660 degrees Celsius, as measured by type K thermocouples.The temperatures are held at these values for 94 hours. The cell iscooled and removed from the press.

Upon opening the capsule after venting of the ammonia, the slot iscovered over and sealed by newly grown gallium nitride crystallinecomposition. The slot is transparent and sealed with high quality newcrystalline composition without any visible cracks, boundary or otherdefects, though a seam/boundary would be expected in the center of theslot.

Example 15

A 1.9 millimeters×5.1 millimeters slot is laser-cut in the center of an8.8 millimeters×15.1 millimeters×0.2 millimeters HVPE gallium nitridecrystal. The gallium nitride seed crystal is hung from a 4 percentopen-area baffle and placed inside a 1.1 inch diameter silver capsule.1.0 grams of NH₄F and 10.03 grams of polycrystalline gallium nitride areadded to the capsule inside a glove box, a lid with a 0.12 inch diameterfill tube is welded to the top of the capsule. The fill tube is attachedto a gas manifold without any exposure of the contents to air and thecapsule is first evacuated and filled with 8.54 grams of NH₃. The filltube is welded shut. The capsule is placed in a cell in a zero-strokehigh-pressure apparatus. The cell is heated until the temperature of thebottom of the capsule is approximately 700 degrees Celsius and thetemperature of the top half of the capsule is approximately 665 degreesCelsius, as measured by type K thermocouples. The temperatures are heldat these values for 60 hr. The cell is cooled and removed from thepress.

Upon opening the capsule after venting of the ammonia, the slot iscovered over by newly grown crystalline gallium nitride, which is clearand nearly colorless. X-ray diffraction studies are performed on thisregion. For the (0002) reflection, the intensity vs. ω (rocking curve)measurement yielded a full width at half maximum (FWHM) of 35arc-seconds. The impurity levels, as determined by calibrated secondaryion mass spectrometry (SIMS), on the gallium surface of the portion ofthe gallium nitride crystalline composition grown in the slot are foundto be: oxygen, 5×10¹⁷ cm⁻³; hydrogen, 3×10¹⁸ cm⁻³; carbon, 4×10¹⁶ cm⁻³;and silicon, 6×10¹⁵ cm⁻³. On the nitrogen surface of the same portion ofthe gallium nitride crystalline composition the corresponding impuritylevels are found to be: oxygen, 4×10¹⁷ cm⁻³; hydrogen, 2×10¹⁸ cm⁻³;carbon, 5×10¹⁶ cm⁻³; and silicon, 2×10¹⁶ cm⁻³.

Example 16

A series of undoped gallium nitride crystalline composition are producedin accordance with an Example process disclosed above. The galliumnitride crystalline compositions produced in sample 1 are undoped,transparent and colorless; in sample 2 are opaque and aresemi-insulating; and in sample 3 are transparent and are p-typeconducting at about room temperature. The gallium nitride crystallinecompositions in samples 4-20 include other compositions as listed inTable 1.

The samples 4-20 are processed into wafers. Such wafer processingincludes polishing, etching, and edge chamfering. The wafers areevaluated as semi-conductor chips in electronics applications. Theelectronics applications include, variously and according to the wafercharacteristics: a light emitting diode, a laser diode, a photodetector,an avalanche photodiode, a p-i-n diode, a metal-semiconductor-metaldiode, a Schottky rectifier, a high-electron mobility transistor, ametal semiconductor field effect transistor, a metal oxide field effecttransistor, a power metal oxide semiconductor field effect transistor, apower metal insulator semiconductor field effect transistor, a bipolarjunction transistor, a metal insulator field effect transistor, aheterojunction bipolar transistor, a power insulated gate bipolartransistor, a power vertical junction field effect transistor, a cascodeswitch, an inner sub-band emitter, a quantum well infraredphotodetector, and a quantum dot infrared photodetector.

A method for forming gallium nitride crystalline composition materialdescribed above enables growth of larger high-quality gallium nitridecrystals. These gallium nitride crystalline compositions may enable thefabrication of electronic and optoelectronic devices having relativelyimproved efficiency, reliability, yield, power performance, breakdownvoltage, and reduced dark current and defect- and trap-induced noise.

TABLE 1 Sample number Other material Sample 4 5 mole % aluminum Sample 55 mole % arsenic Sample 6 5 mole % boron Sample 7 5 mole % indium Sample8 5 mole % phosphorus Sample 9 2.5 mole % aluminum 2 mole % indiumSample 10 2.5 mole % aluminum 2 mole % phosphorus Sample 11 2.5 mole %aluminum 2.5 mole % arsenic Sample 12 2.5 mole % indium 2.5 mole %phosphorus Sample 13 2.5 mole % indium 2.5 mole % arsenic Sample 14 0.5mole % phosphorus 3.5 mole % arsenic Sample 15 3.5 mole % aluminum 1.5mole % boron Sample 16 2.5 mole % arsenic 2.5 mole % boron Sample 17 0.5mole % boron 0.05 mole % indium Sample 18 0.05 mole % boron 0.05 mole %phosphorus Sample 19 1.25 mole % phosphorus 1.25 mole % arsenic 1.25mole % indium 1.25 mole % aluminum Sample 20a 1 mole % aluminum 1 mole %arsenic 1 mole % boron 1 mole % indium 1 mole % phosphorus Sample 20b 3mole % aluminum 0.02 mole % arsenic 0.01 mole % boron 0.5 mole % indium0.01 mole % phosphorus

Reference is made to substances, components, or ingredients in existenceat the time just before first contacted, formed in situ, blended, ormixed with one or more other substances, components, or ingredients inaccordance with the present disclosure. A substance, component oringredient identified as a reaction product, resulting mixture, or thelike may gain an identity, property, or character through a chemicalreaction or transformation during the course of contacting, in situformation, blending, or mixing operation if conducted in accordance withthis disclosure with the application of common sense and the ordinaryskill of one in the relevant art (e.g., chemist). The transformation ofchemical reactants or starting materials to chemical products or finalmaterials is a continually evolving process, independent of the speed atwhich it occurs. Accordingly, as such a transformative process is inprogress there may be a mix of starting and final materials, as well asintermediate species that may be, depending on their kinetic lifetime,easy or difficult to detect with current analytical techniques known tothose of ordinary skill in the art.

Reactants and components referred to by chemical name or formula in thespecification or claims hereof, whether referred to in the singular orplural, may be identified as they exist prior to coming into contactwith another substance referred to by chemical name or chemical type(e.g., another reactant or a solvent). Preliminary and/or transitionalchemical changes, transformations, or reactions, if any, that take placein the resulting mixture, solution, or reaction medium may be identifiedas intermediate species, master batches, and the like, and may haveutility distinct from the utility of the reaction product or finalmaterial. Other subsequent changes, transformations, or reactions mayresult from bringing the specified reactants and/or components togetherunder the conditions called for pursuant to this disclosure. In theseother subsequent changes, transformations, or reactions the reactants,ingredients, or the components to be brought together may identify orindicate the reaction product or final material.

The embodiments described herein are examples of compositions,structures, systems and methods having elements corresponding to theelements of the invention recited in the claims. This writtendescription may enable those of ordinary skill in the art to make anduse embodiments having alternative elements that likewise correspond tothe elements of the invention recited in the claims. The scope of theinvention includes compositions, structures, systems and methods that donot differ from the literal language of the claims, and further includesother structures, systems and methods with insubstantial differencesfrom the literal language of the claims. While only certain features andembodiments have been illustrated and described herein, manymodifications and changes may occur to one of ordinary skill in therelevant art. The appended claims cover all such modifications andchanges.

What is claimed is:
 1. A crystal comprising: a single crystal of GaNhaving a wurtzite structure; and fluorine having a concentration of atleast 0.04 ppm.
 2. The crystal of claim 1, comprising at most 1 ppmfluorine.
 3. The crystal according to claim 1, having an infraredabsorption peak at 3175 cm⁻¹, with an absorbance per unit thickness ofgreater than 0.01 cm⁻¹.
 4. The crystal according to claim 1, having anoxygen impurity level of less than 1×10¹⁹ cm⁻³.
 5. The crystal accordingto claim 1, having an oxygen impurity level of less than 3×10¹⁸ cm⁻³. 6.The crystal according to claim 1, having a maximum dimension of at least2 millimeters.
 7. The crystal according to claim 1, having a dislocationdensity below 10⁶ cm⁻².
 8. The crystal according to claim 7, having adislocation density below 10⁴ cm⁻².
 9. The crystal according to claim 1,wherein the single crystal is free of tilt boundaries.
 10. The crystalaccording to claim 1, wherein the single crystal comprises one of n-type and p-type semiconductor and is optically transparent, with anoptical absorption coefficient below 100 cm⁻¹ at wavelengths between 700and 465 nm.
 11. The crystal of claim 10, wherein the optical absorptioncoefficient is below 5 cm⁻¹ at wavelengths between 465 and 700 nm. 12.The crystal according to claim 1, further comprising a dopant comprisingone or more of Be, C, Mg, Si, H, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu,Zn, Ge, Zr, Hf, or a rare earth metal.
 13. The crystal according toclaim 12, wherein the dopant is present at a concentration in a rangefrom 1×10¹⁶ per cubic centimeter to 1×10²¹ per cubic centimeter.
 14. Thecrystal according to claim 1, wherein the crystal is magnetic, isluminescent, or is both magnetic and luminescent.
 15. The crystalaccording to claim 1, wherein the crystal is characterized by aphotoluminescence spectrum which peaks at a photon energy of in a rangeof from about 3.38 to about 3.41 eV at a temperature of 300 K.
 16. Acrystal made from a process comprising: providing a nucleation center ina first region of a chamber; providing a gallium nitride source materialin a second region of the chamber; providing a solvent in the chamber,the solvent comprising nitrogen; providing a mineralizer in the chamber,the mineralizer comprising fluorine; and heating the chamber andperforming ammonothermal growth of GaN on the nucleation center underconditions to grow a single crystal of GaN having a wurtzite and havinga concentration of fluorine of at least 0.04 ppm.
 17. The crystal ofclaim 16, wherein the mineralizer comprises at least one of hydrogenfluoride. ammonium fluoride, gallium trifluoride, or a compound producedby a chemical reaction between at least one of hydrogen fluoride,ammonium fluoride, gallium trifluoride, and one or more of ammonia(NH₃), gallium, or gallium nitride.
 18. The crystal of claim 16, whereinthe process further comprises heating the chamber to a temperature of atleast 550 degrees Celsius.
 19. The crystal of claim 16, wherein theprocess further comprises generating and holding a first temperaturedistribution such that the solvent is supersaturated in the first regionof the chamber and such that there is a first temperature gradientbetween the nucleation center and the gallium nitride source materialsuch that gallium nitride crystalline composition grows on thenucleation center; and generating a second temperature distribution inthe chamber such that the solvent is supersaturated in the first regionof the chamber and such that there is a second temperature gradientbetween the nucleation center and the gallium nitride source materialsuch that gallium nitride crystalline composition grows on thenucleation center, wherein the second temperature gradient is larger inmagnitude than the first temperature gradient.
 20. The crystal of claim16, wherein the process further comprises: generating and holding afirst temperature distribution such that there is a first temperaturegradient between the nucleation center and the gallium nitride sourcematerial; and generating a second temperature distribution in thechamber such that the solvent is supersaturated in the first region ofthe chamber and such that there is a second temperature gradient betweenthe nucleation center and the gallium nitride source material such thatgallium nitride crystalline composition grows on the nucleation center,wherein the first temperature gradient is zero or opposite in sign fromthe second temperature gradient.